Gitのpre-commit.sampleにある4b825dc642cb6eb9a060e54bf8d69288fbee4904とは？

.git/hooks/pre-commit.sample

~~
# An example hook script to verify what is about to be committed.
# Called by "git commit" with no arguments.  The hook should
# exit with non-zero status after issuing an appropriate message if
# it wants to stop the commit.
#
# To enable this hook, rename this file to "pre-commit".

if git rev-parse --verify HEAD >/dev/null 2>&1
then
    against=HEAD
else
    # Initial commit: diff against an empty tree object
    against=4b825dc642cb6eb9a060e54bf8d69288fbee4904
fi
~~

にあるマジックナンバー的なSHA値
4b825dc642cb6eb9a060e54bf8d69288fbee4904
はempty treeを表す特殊なSHA値とのこと

参考

git's semi-secret empty tree - Stack Overflow