We do not document COMMIT_EDITMSG at all, but users may want
to know about it for two reasons:
1. They may want to tell their editor to configure itself
for formatting a commit message.
2. If a commit is aborted by an error, the user may want
to recover the commit message they typed.
Let's put a note in git-commit(1).
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>