builtin/am: honor --signoff also when --rebasing

builtin/am: honor --signoff also when --rebasing

Signoff is handled in parse_mail(), but not in parse_mail_rebasing(),
since the latter is only used when git-rebase calls git-am with the
--rebasing option, and --signoff is never passed in this case.

In order to introduce (in the upcoming commits) support for
`git-rebase --signoff`, we must make git-am pay attention to it also
in the rebase case. This can be done by moving the conditional
addition of the signoff from parse_mail() to the caller am_run(),
after either of the parse_mail*() functions were called.

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>