builtin-am: bypass git-mailinfo when --rebasing

builtin-am: bypass git-mailinfo when --rebasing

Since 5e835ca (rebase: do not munge commit log message, 2008-04-16),
git am --rebasing no longer gets the commit log message from the patch,
but reads it directly from the commit identified by the "From " header
line.

Since 43c2325 (am: use get_author_ident_from_commit instead of mailinfo
when rebasing, 2010-06-16), git am --rebasing also gets the author name,
email and date directly from the commit.

Since 0fbb95d (am: don't call mailinfo if $rebasing, 2012-06-26), git am
--rebasing does not use git-mailinfo to get the patch body, but rather
generates it directly from the commit itself.

The above 3 commits introduced a separate parse_mail() code path in
git-am.sh's --rebasing mode that bypasses git-mailinfo. Re-implement
this code path in builtin/am.c as parse_mail_rebase().

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>