Gitの動作を理解するために、Gitのコマンドを実際に試して、結果を見てみました。
1つの記事内で一連のGitコマンドが完結しているので、これら一連のコマンドを順に実行させて結果を見ることで、一連のGitの動作を実際に体感でき、一通り独習することが可能です。
※前回記事のリポジトリ状態からの続きになっています。
前回記事へ	目次へ：Git関連記事のまとめページ	次回記事へ
実行例

(※一度pushしたコミット履歴を、reset等で意図的にコミットを削除すると、他人側ではどうなるかを試行)

-----

(※試行用にlocal-repo3を作成)

cd /test-space
mkdir local-repo3
cd /test-space/local-repo3

git init
git remote add origin "D:/test-space/remote-repo1.git"

git log
↓
結果: 
fatal: your current branch 'master' does not have any commits yet

-----

(※リモートリポジトリから履歴情報を取得して環境準備)
git fetch origin
git checkout branch-R1
git log --oneline --graph --all
↓
結果: 
* 1247fa9 (origin/master) message R4
* fab68d8 (origin/branch-R2) message R3
* c69a305 message R1
* 94996a0 message4
| * e009b2d (HEAD -> branch-R1, origin/branch-R1) message R2
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

-----

(※試行用にlocal-repo4をコピーして作成)

xcopy /H /E "D:\test-space\local-repo3" "D:\test-space\local-repo4"

cd /test-space/local-repo4
git log --oneline --graph --all
↓
結果: 
* 1247fa9 (origin/master) message R4
* fab68d8 (origin/branch-R2) message R3
* c69a305 message R1
* 94996a0 message4
| * e009b2d (HEAD -> branch-R1, origin/branch-R1) message R2
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

(※local-repo3とlocal-repo4の間で以降試行する)

-----

(※local-repo3でpushした内容をlocal-repo4で取得後に、pushを戻してしまう時)

cd /test-space/local-repo3

(※ひとまず適当にbranch-R1で変更の操作)
echo Sample-Added-R5 >> test1.txt
git commit -a -m "message R5"
↓
結果: 
[branch-R1 3e758f9] message R5
 1 file changed, 1 insertion(+)

git log --oneline --graph --all
↓
結果: 
* 3e758f9 (HEAD -> branch-R1) message R5
* e009b2d (origin/branch-R1) message R2
| * 1247fa9 (origin/master) message R4
| * fab68d8 (origin/branch-R2) message R3
| * c69a305 message R1
| * 94996a0 message4
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

git push origin branch-R1
↓
結果: 
Enumerating objects: 5, done.
Counting objects: 100% (5/5), done.
Delta compression using up to 8 threads
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 303 bytes | 303.00 KiB/s, done.
Total 3 (delta 1), reused 0 (delta 0), pack-reused 0
To D:/test-space/remote-repo1.git
   e009b2d..3e758f9  branch-R1 -> branch-R1

cd /test-space/local-repo4

git log --oneline --graph --all
↓
結果: 
* 1247fa9 (origin/master) message R4
* fab68d8 (origin/branch-R2) message R3
* c69a305 message R1
* 94996a0 message4
| * e009b2d (HEAD -> branch-R1, origin/branch-R1) message R2
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

git fetch origin branch-R1
↓
結果: 
remote: Enumerating objects: 5, done.
remote: Counting objects: 100% (5/5), done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 3 (delta 1), reused 0 (delta 0), pack-reused 0
Unpacking objects: 100% (3/3), 283 bytes | 1024 bytes/s, done.
From D:/test-space/remote-repo1
 * branch            branch-R1  -> FETCH_HEAD
   e009b2d..3e758f9  branch-R1  -> origin/branch-R1

git log --oneline --graph --all
↓
結果: 
* 3e758f9 (origin/branch-R1) message R5
* e009b2d (HEAD -> branch-R1) message R2
| * 1247fa9 (origin/master) message R4
| * fab68d8 (origin/branch-R2) message R3
| * c69a305 message R1
| * 94996a0 message4
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

git pull origin branch-R1
↓
結果: 
From D:/test-space/remote-repo1
 * branch            branch-R1  -> FETCH_HEAD
Updating e009b2d..3e758f9
Fast-forward
 test1.txt | 1 +
 1 file changed, 1 insertion(+)

git log --oneline --graph --all
↓
結果: 
* 3e758f9 (HEAD -> branch-R1, origin/branch-R1) message R5
* e009b2d message R2
| * 1247fa9 (origin/master) message R4
| * fab68d8 (origin/branch-R2) message R3
| * c69a305 message R1
| * 94996a0 message4
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

(※local-repo4側でもmessage R5の所まで更新される)

cd /test-space/local-repo3

(※前回のpushの操作を戻す)
git reset --hard e009b2d

git log --oneline --graph --all
↓
結果: 
* 3e758f9 (origin/branch-R1) message R5
* e009b2d (HEAD -> branch-R1) message R2
| * 1247fa9 (origin/master) message R4
| * fab68d8 (origin/branch-R2) message R3
| * c69a305 message R1
| * 94996a0 message4
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

※位置を戻したbranch-R1ブランチをリモートリポジトリへpush送信 (過去側に進めるpush)
git push -f origin branch-R1
↓
結果: 
Total 0 (delta 0), reused 0 (delta 0), pack-reused 0
To D:/test-space/remote-repo1.git
 + 3e758f9...e009b2d branch-R1 -> branch-R1 (forced update)

git log --oneline --graph --all
↓
結果: 
* 1247fa9 (origin/master) message R4
* fab68d8 (origin/branch-R2) message R3
* c69a305 message R1
* 94996a0 message4
| * e009b2d (HEAD -> branch-R1, origin/branch-R1) message R2
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

(※message R5のコミット履歴が消える)

cd /test-space/local-repo4

git log --oneline --graph --all
↓
結果: 
* 3e758f9 (HEAD -> branch-R1, origin/branch-R1) message R5
* e009b2d message R2
| * 1247fa9 (origin/master) message R4
| * fab68d8 (origin/branch-R2) message R3
| * c69a305 message R1
| * 94996a0 message4
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

git fetch origin branch-R1
↓
結果: 
From D:/test-space/remote-repo1
 * branch            branch-R1  -> FETCH_HEAD
 + 3e758f9...e009b2d branch-R1  -> origin/branch-R1  (forced update)

git log --oneline --graph --all
↓
結果: 
* 3e758f9 (HEAD -> branch-R1) message R5
* e009b2d (origin/branch-R1) message R2
| * 1247fa9 (origin/master) message R4
| * fab68d8 (origin/branch-R2) message R3
| * c69a305 message R1
| * 94996a0 message4
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

(※ローカルブランチbranch-R1だけ1つ先に進んだ状態のまま)

git pull origin branch-R1
↓
結果: 
From D:/test-space/remote-repo1
 * branch            branch-R1  -> FETCH_HEAD
Already up to date.

(※ローカルブランチbranch-R1の方が1つ先に進んでいるので、pullでは更新なしとなる模様)

git log --oneline --graph --all
↓
結果: 
* 3e758f9 (HEAD -> branch-R1) message R5
* e009b2d (origin/branch-R1) message R2
| * 1247fa9 (origin/master) message R4
| * fab68d8 (origin/branch-R2) message R3
| * c69a305 message R1
| * 94996a0 message4
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

-----

(※ここから更に、リモートブランチorigin/branch-R1が違う枝の方へ進んだ時)

cd /test-space/local-repo3

(※branch-R1を違う枝の方へ進める操作)
echo Sample-Added-R6 >> test1.txt
git commit -a -m "message R6"
↓
結果: 
[branch-R1 3a9cf58] message R6
 1 file changed, 1 insertion(+)

git log --oneline --graph --all
↓
結果: 
* 3a9cf58 (HEAD -> branch-R1) message R6
* e009b2d (origin/branch-R1) message R2
| * 1247fa9 (origin/master) message R4
| * fab68d8 (origin/branch-R2) message R3
| * c69a305 message R1
| * 94996a0 message4
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

git push origin branch-R1
↓
結果: 
Enumerating objects: 5, done.
Counting objects: 100% (5/5), done.
Delta compression using up to 8 threads
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 302 bytes | 302.00 KiB/s, done.
Total 3 (delta 1), reused 0 (delta 0), pack-reused 0
To D:/test-space/remote-repo1.git
   e009b2d..3a9cf58  branch-R1 -> branch-R1

cd /test-space/local-repo4

git log --oneline --graph --all
↓
結果: 
* 3e758f9 (HEAD -> branch-R1) message R5
* e009b2d (origin/branch-R1) message R2
| * 1247fa9 (origin/master) message R4
| * fab68d8 (origin/branch-R2) message R3
| * c69a305 message R1
| * 94996a0 message4
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

git fetch origin branch-R1
↓
結果: 
remote: Enumerating objects: 5, done.
remote: Counting objects: 100% (5/5), done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 3 (delta 1), reused 0 (delta 0), pack-reused 0
Unpacking objects: 100% (3/3), 282 bytes | 0 bytes/s, done.
From D:/test-space/remote-repo1
 * branch            branch-R1  -> FETCH_HEAD
   e009b2d..3a9cf58  branch-R1  -> origin/branch-R1

git log --oneline --graph --all
↓
結果: 
* 3a9cf58 (origin/branch-R1) message R6
| * 3e758f9 (HEAD -> branch-R1) message R5
|/
* e009b2d message R2
| * 1247fa9 (origin/master) message R4
| * fab68d8 (origin/branch-R2) message R3
| * c69a305 message R1
| * 94996a0 message4
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

git pull -f origin branch-R1
↓
結果: 
From D:/test-space/remote-repo1
 * branch            branch-R1  -> FETCH_HEAD
hint: You have divergent branches and need to specify how to reconcile them.
hint: You can do so by running one of the following commands sometime before
hint: your next pull:
hint:
hint:   git config pull.rebase false  # merge
hint:   git config pull.rebase true   # rebase
hint:   git config pull.ff only       # fast-forward only
hint:
hint: You can replace "git config" with "git config --global" to set a default
hint: preference for all repositories. You can also pass --rebase, --no-rebase,
hint: or --ff-only on the command line to override the configured default per
hint: invocation.
fatal: Need to specify how to reconcile divergent branches.

(※リモートorigin/branch-R1とローカルbranch-R1の間で違う枝になっているので、pull出来ない状態)

(※local-repo4側では、知らないうちに違う枝に取り残されて、何もしていないのにmergeが必要になってしまう)

git merge origin/branch-R1
↓
結果: 
Auto-merging test1.txt
CONFLICT (content): Merge conflict in test1.txt
Automatic merge failed; fix conflicts and then commit the result.

git diff
↓
結果: 
diff --cc test1.txt
index 874d124,5de92d6..0000000
--- a/test1.txt
+++ b/test1.txt
@@@ -2,4 -2,4 +2,8 @@@ Sample-Added-1
  Sample-Added-12
  Sample-Added-13
  Sample-Added-R2
++<<<<<<< HEAD
 +Sample-Added-R5
++=======
+ Sample-Added-R6
++>>>>>>> origin/branch-R1

(※コンフリクトを手動で解消)
echo Sample-Added-11 > test1.txt
echo Sample-Added-12 >> test1.txt
echo Sample-Added-13 >> test1.txt
echo Sample-Added-R2 >> test1.txt
echo Sample-Added-R5 >> test1.txt
echo Sample-Added-R6 >> test1.txt
type test1.txt
↓
結果: 
Sample-Added-11
Sample-Added-12
Sample-Added-13
Sample-Added-R2
Sample-Added-R5
Sample-Added-R6

git add .
git status -s
↓
結果: 
M  test1.txt

※マージ処置をコミット
git commit --no-edit
↓
結果: 
[branch-R1 f85522d] Merge remote-tracking branch 'origin/branch-R1' into branch-R1

git log --oneline --graph --all
↓
結果: 
*   f85522d (HEAD -> branch-R1) Merge remote-tracking branch 'origin/branch-R1' into branch-R1
|\
| * 3a9cf58 (origin/branch-R1) message R6
* | 3e758f9 message R5
|/
* e009b2d message R2
| * 1247fa9 (origin/master) message R4
| * fab68d8 (origin/branch-R2) message R3
| * c69a305 message R1
| * 94996a0 message4
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

git push origin branch-R1
git log --oneline --graph --all
↓
結果: 
*   f85522d (HEAD -> branch-R1, origin/branch-R1) Merge remote-tracking branch 'origin/branch-R1' into branch-R1
|\
| * 3a9cf58 message R6
* | 3e758f9 message R5
|/
* e009b2d message R2
| * 1247fa9 (origin/master) message R4
| * fab68d8 (origin/branch-R2) message R3
| * c69a305 message R1
| * 94996a0 message4
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

(※回復は出来るが、面倒なので、苦情の元となる)

-----

(※pushで公開済みのbranch-R1ブランチを、local-repo3でリベースしてpushしてしまう時)

cd /test-space/local-repo3

git pull origin
↓
結果: 
remote: Enumerating objects: 10, done.
remote: Counting objects: 100% (10/10), done.
remote: Compressing objects: 100% (6/6), done.
remote: Total 6 (delta 2), reused 0 (delta 0), pack-reused 0
Unpacking objects: 100% (6/6), 603 bytes | 0 bytes/s, done.
From D:/test-space/remote-repo1
   3a9cf58..f85522d  branch-R1  -> origin/branch-R1
Updating 3a9cf58..f85522d
Fast-forward
 test1.txt | 1 +
 1 file changed, 1 insertion(+)

git log --oneline --graph --all
↓
結果: 
*   f85522d (HEAD -> branch-R1, origin/branch-R1) Merge remote-tracking branch 'origin/branch-R1' into branch-R1
|\
| * 3a9cf58 message R6
* | 3e758f9 message R5
|/
* e009b2d message R2
| * 1247fa9 (origin/master) message R4
| * fab68d8 (origin/branch-R2) message R3
| * c69a305 message R1
| * 94996a0 message4
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

git rebase origin/master
↓
結果: 
Auto-merging test1.txt
CONFLICT (content): Merge conflict in test1.txt
error: could not apply e009b2d... message R2
hint: Resolve all conflicts manually, mark them as resolved with
hint: "git add/rm <conflicted_files>", then run "git rebase --continue".
hint: You can instead skip this commit: run "git rebase --skip".
hint: To abort and get back to the state before "git rebase", run "git rebase --abort".
Could not apply e009b2d... message R2

git diff
↓
結果: 
diff --cc test1.txt
index 4222b46,f4e41fe..0000000
--- a/test1.txt
+++ b/test1.txt
@@@ -1,7 -1,4 +1,11 @@@
  Sample-Added-11
  Sample-Added-12
  Sample-Added-13
++<<<<<<< HEAD
 +Sample-Added-14
 +Sample-Added-R1
 +Sample-Added-R3
 +Sample-Added-R4
++=======
+ Sample-Added-R2
++>>>>>>> e009b2d (message R2)

(※コンフリクトを手動で解消)
echo Sample-Added-11 > test1.txt
echo Sample-Added-12 >> test1.txt
echo Sample-Added-13 >> test1.txt
echo Sample-Added-R1+2+3+4 >> test1.txt
type test1.txt
↓
結果: 
Sample-Added-11
Sample-Added-12
Sample-Added-13
Sample-Added-R1+2+3+4

git add .
git rebase --continue

git rebase --continue
↓
結果: 
Auto-merging test1.txt
CONFLICT (content): Merge conflict in test1.txt
error: could not apply 3e758f9... message R5

git diff
↓
結果: 
diff --cc test1.txt
index 08cdc60,874d124..0000000
--- a/test1.txt
+++ b/test1.txt
@@@ -1,4 -1,5 +1,9 @@@
  Sample-Added-11
  Sample-Added-12
  Sample-Added-13
++<<<<<<< HEAD
 +Sample-Added-R1+2+3+4
++=======
+ Sample-Added-R2
+ Sample-Added-R5
++>>>>>>> 3e758f9 (message R5)

(※コンフリクトを手動で解消)
echo Sample-Added-11 > test1.txt
echo Sample-Added-12 >> test1.txt
echo Sample-Added-13 >> test1.txt
echo Sample-Added-R1+2+3+4+5 >> test1.txt
type test1.txt
↓
結果: 
Sample-Added-11
Sample-Added-12
Sample-Added-13
Sample-Added-R1+2+3+4+5

git add .
git rebase --continue

git rebase --continue
↓
結果: 
Auto-merging test1.txt
CONFLICT (content): Merge conflict in test1.txt
error: could not apply 3a9cf58... message R6

git diff
↓
結果: 
diff --cc test1.txt
index 5e4aa50,5de92d6..0000000
--- a/test1.txt
+++ b/test1.txt
@@@ -1,4 -1,5 +1,9 @@@
  Sample-Added-11
  Sample-Added-12
  Sample-Added-13
++<<<<<<< HEAD
 +Sample-Added-R1+2+3+4+5
++=======
+ Sample-Added-R2
+ Sample-Added-R6
++>>>>>>> 3a9cf58 (message R6)

(※コンフリクトを手動で解消)
echo Sample-Added-11 > test1.txt
echo Sample-Added-12 >> test1.txt
echo Sample-Added-13 >> test1.txt
echo Sample-Added-R1+2+3+4+5+6 >> test1.txt
type test1.txt
↓
結果: 
Sample-Added-11
Sample-Added-12
Sample-Added-13
Sample-Added-R1+2+3+4+5+6

git add .
git rebase --continue

git rebase --continue
↓
結果: 
Successfully rebased and updated refs/heads/branch-R1.

git log --oneline --graph --all
↓
結果: 
* a605230 (HEAD -> branch-R1) message R6
* 5f95c9d message R5
* 1002a3a message R2
* 1247fa9 (origin/master) message R4
* fab68d8 (origin/branch-R2) message R3
* c69a305 message R1
* 94996a0 message4
| *   f85522d (origin/branch-R1) Merge remote-tracking branch 'origin/branch-R1' into branch-R1
| |\
| | * 3a9cf58 message R6
| * | 3e758f9 message R5
| |/
| * e009b2d message R2
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

(※origin/masterからmessage R2, R5, R6がリベースで追加されている状態)

(※リベースしたものをpush送信)
git push origin branch-R1
↓
結果: 
To D:/test-space/remote-repo1.git
 ! [rejected]        branch-R1 -> branch-R1 (non-fast-forward)
error: failed to push some refs to 'D:/test-space/remote-repo1.git'

git push -f origin branch-R1
↓
結果: 
Enumerating objects: 11, done.
Counting objects: 100% (11/11), done.
Delta compression using up to 8 threads
Compressing objects: 100% (9/9), done.
Writing objects: 100% (9/9), 867 bytes | 289.00 KiB/s, done.
Total 9 (delta 3), reused 0 (delta 0), pack-reused 0
To D:/test-space/remote-repo1.git
 + f85522d...a605230 branch-R1 -> branch-R1 (forced update)

(※強制的なpushで実施可)

git log --oneline --graph --all
↓
結果: 
* a605230 (HEAD -> branch-R1, origin/branch-R1) message R6
* 5f95c9d message R5
* 1002a3a message R2
* 1247fa9 (origin/master) message R4
* fab68d8 (origin/branch-R2) message R3
* c69a305 message R1
* 94996a0 message4
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

(※枝分かれ・マージしていた履歴が消えている)

cd /test-space/local-repo4

git log --oneline --graph --all
↓
結果: 
*   f85522d (HEAD -> branch-R1, origin/branch-R1) Merge remote-tracking branch 'origin/branch-R1' into branch-R1
|\
| * 3a9cf58 message R6
* | 3e758f9 message R5
|/
* e009b2d message R2
| * 1247fa9 (origin/master) message R4
| * fab68d8 (origin/branch-R2) message R3
| * c69a305 message R1
| * 94996a0 message4
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

git fetch origin
↓
結果: 
remote: Enumerating objects: 11, done.
remote: Counting objects: 100% (11/11), done.
remote: Compressing objects: 100% (9/9), done.
remote: Total 9 (delta 3), reused 0 (delta 0), pack-reused 0
Unpacking objects: 100% (9/9), 847 bytes | 3.00 KiB/s, done.
From D:/test-space/remote-repo1
 + f85522d...a605230 branch-R1  -> origin/branch-R1  (forced update)

git log --oneline --graph --all
↓
結果: 
* a605230 (origin/branch-R1) message R6
* 5f95c9d message R5
* 1002a3a message R2
* 1247fa9 (origin/master) message R4
* fab68d8 (origin/branch-R2) message R3
* c69a305 message R1
* 94996a0 message4
| *   f85522d (HEAD -> branch-R1) Merge remote-tracking branch 'origin/branch-R1' into branch-R1
| |\
| | * 3a9cf58 message R6
| * | 3e758f9 message R5
| |/
| * e009b2d message R2
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

(※local-repo4側では、知らないうちにorigin/branch-R1が、branch-R1系列から別のブランチ系列に移ってしまった)

git pull origin branch-R1
↓
結果: 
From D:/test-space/remote-repo1
 * branch            branch-R1  -> FETCH_HEAD
fatal: Need to specify how to reconcile divergent branches.

git log --oneline --graph --all
↓
結果: 
* a605230 (origin/branch-R1) message R6
* 5f95c9d message R5
* 1002a3a message R2
* 1247fa9 (origin/master) message R4
* fab68d8 (origin/branch-R2) message R3
* c69a305 message R1
* 94996a0 message4
| *   f85522d (HEAD -> branch-R1) Merge remote-tracking branch 'origin/branch-R1' into branch-R1
| |\
| | * 3a9cf58 message R6
| * | 3e758f9 message R5
| |/
| * e009b2d message R2
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

(※リモートorigin/branch-R1とローカルbranch-R1の間で違う枝になっているので、pull出来ない状態)

(※local-repo4側では、知らないうちに違う枝に取り残されて、何もしていないのにmergeが必要になってしまう)

-----

(※リモートリポジトリを元に戻す)

git push -f origin e009b2d:branch-R1

git checkout origin/master
git branch -D branch-R1
git log --oneline --graph --all
↓
結果: 
* 1247fa9 (HEAD, origin/master) message R4
* fab68d8 (origin/branch-R2) message R3
* c69a305 message R1
* 94996a0 message4
| * e009b2d (origin/branch-R1) message R2
|/
* 20bbab7 message3
* 00cad71 message2
* 4ace194 message1

-----

(※local-repo3,4を削除)
cd /test-space
rd /s /q "/test-space/local-repo3"
rd /s /q "/test-space/local-repo4"
環境

Windows 10、PortableGit-2.40.0-64-bitを使用、全てローカルPC上で実施、GitHub等は不使用。