Hm, could it be that "git diff --cached" compares the index to the old version of the commit, rather than to the parent (of both the old and new version) of the commit, when making an amend?
In that case, how can the hook know, and compare against the right version? Of course, the hook doesn't get any arguments at all, so it has to guess what's going on, as per usual in git land... As a kluge I could check everything _again_ in the post-commit hook. At that time it is at least obivious (I hope!!!) what the new commit contains and which its parents are.
