commit: give advice on empty amend

commit: give advice on empty amend

We generally disallow empty commits with "git commit". The
output produced by the wt_status functions is generally
sufficient to explain what happened.

With --amend commits, however, things are a little more
confusing. We would create an empty commit not if you
actually have staged changes _now_, but if your staged
changes match HEAD^. In this case, it is not immediately
obvious why "git commit" claims no changes, but "git status"
does not. Furthermore, we should point the user in the
direction of git reset, which would eliminate the empty
commit entirely.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>