commit --amend: invoke post-rewrite hook

commit --amend: invoke post-rewrite hook

The rough structure of run_rewrite_hook() comes from
run_receive_hook() in receive-pack.

We introduce a --no-post-rewrite option and use it to avoid the hook
when called from git-rebase -i 'edit'.  The next patch will add full
support in git-rebase, and we only want to invoke the hook once.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>