Another useful thing if you need to backdate commits for whatever reason are the GIT_AUTHOR_DATE and the GIT_COMMITTER_DATE environment variables, upon executing git-commit they'll override these fields in the commit to whatever date, time and timezone you specify. I use this sometimes when I'm making some previously private work public, and am redoing the commit history to make more logical sense to others who may read it.
Also useful are git-fast-export and git-fast-import, if you really need to delve into the inner details of a commit. For example, I had three separate but related git repos that I needed to merge, so I created a new repo with separate branches to hold each repo, merged everything manually, committed that to a new branch, then used export/import to edit the commit to have the tips of the three other branches as its ancestors. Maybe there's a better way to do this with other git commands but I found it easier just to delve in and edit the commit data manually.