commit: move post-rewrite code to libgit

commit: move post-rewrite code to libgit

Move run_rewrite_hook() from bulitin/commit.c to sequencer.c so it can
be shared with other commands and add a new function
commit_post_rewrite() based on the code in builtin/commit.c that
encapsulates rewriting notes and running the post-rewrite hook. Once
the sequencer learns how to create commits without forking 'git
commit' these functions will be used when squashing commits.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>