c.txt と d.txt を一緒にcommitしちゃってる！
分けたい！分けたい！（大事なことなので2回言いました)

❯ git log
commit 3320bf4a4c8d7165e30282cdcd6795602d00bdc3 (HEAD -> master)
Author: yamazoon <xxxx@gmail.com>
Date:   Tue Apr 2 02:06:54 2019 +0900

    add b.txt & c.txt

commit d5f820d0fd366702fa388ebad947b76f9c7ed1b1
Author: yamazoon <xxxx@gmail.com>
Date:   Tue Apr 2 02:06:32 2019 +0900

    add b.txt

commit 540edae6ace436c9c1817afd5d549fc422a5d34d
Author: yamazoon <xxxx@gmail.com>
Date:   Tue Apr 2 02:06:25 2019 +0900

    add a.txt

私は、git rebase -i あなたの履歴を守ります (HEAD~1で直近のコミット履歴を編集します)

❯ git rebase -i HEAD~1

エディタ上で先頭のpickをeditに変えて保存します

before


pick bbaedb1 add c.txt & d.txt

# Rebase 76a09a7..bbaedb1 onto 76a09a7 (1 command)
# 〜略〜

after


edit bbaedb1 add c.txt & d.txt

# Rebase 76a09a7..bbaedb1 onto 76a09a7 (1 command)
# 〜略〜

1つ前のコミットに戻します(zshなので^を¥でエスケープしています)

❯ git reset HEAD\^

Untrackedになっています

❯ git status
interactive rebase in progress; onto d5f820d
Last command done (1 command done):
   edit 3320bf4 add b.txt & c.txt
No commands remaining.
You are currently editing a commit while rebasing branch 'master' on 'd5f820d'.
  (use "git commit --amend" to amend the current commit)
  (use "git rebase --continue" once you are satisfied with your changes)

Untracked files:
  (use "git add <file>..." to include in what will be committed)

	c.txt
	d.txt

nothing added to commit but untracked files present (use "git add" to track)

1つずつコミットします (c.txt)

❯ git add c.txt
❯ git commit c.txt -m "add c.txt"

1つずつコミットします (d.txt)

❯ git add d.txt
❯ git commit d.txt -m "add d.txt"

git rebase -i を終了します

❯ git rebase --continue
Successfully rebased and updated refs/heads/master.

❯ git log
commit 0478f9e80bc4f4a11683ef9e1271f2dda0f56cbf (HEAD -> master)
Author: yamazoon <xxxx@gmail.com>
Date:   Tue Apr 2 02:27:43 2019 +0900

    d
commit 3320bf4a4c8d7165e30282cdcd6795602d00bdc3 (HEAD -> master)
Author: yamazoon <xxxx@gmail.com>
Date:   Tue Apr 2 02:06:54 2019 +0900

    add b.txt & c.txt

commit d5f820d0fd366702fa388ebad947b76f9c7ed1b1
Author: yamazoon <xxxx@gmail.com>
Date:   Tue Apr 2 02:06:32 2019 +0900

    add b.txt

commit 540edae6ace436c9c1817afd5d549fc422a5d34d
Author: yamazoon <xxxx@gmail.com>
Date:   Tue Apr 2 02:06:25 2019 +0900

    add a.txt

reflogすると、履歴の編集を(start)と(finish)の間に書いてくれています

❯ git reflog
21d9934 (HEAD -> master) HEAD@{0}: rebase -i (finish): returning to refs/heads/master
21d9934 (HEAD -> master) HEAD@{1}: commit: add d.txt
3a322e7 HEAD@{2}: commit: add c.txt
d5f820d HEAD@{3}: reset: moving to HEAD^
3320bf4 HEAD@{4}: rebase -i: fast-forward
d5f820d HEAD@{5}: rebase -i (start): checkout HEAD~1
3320bf4 HEAD@{6}: commit: add b.txt & c.txt
d5f820d HEAD@{7}: commit: add b.txt
540edae HEAD@{8}: commit (initial): add a.txt

疲れたので一旦おしまい

補足：もちろん、git logも「add c.txt」と「add d.txt」で分かれてコミットされています