To see the diff for a particular
COMMIT hash, where
COMMIT is the hash of the commit:
git diff COMMIT~ COMMIT will show you the difference between that
COMMIT's ancestor and the
COMMIT. See the man pages for git diff for details about the command and gitrevisions about the
~ notation and its friends.
git show COMMIT will do something very similar. (The commit's data, including its diff - but not for merge commits.) See the git show manpage.
git diff COMMIT will show you the difference between that
COMMIT and the head.)