am: let --signoff override --no-signoff

am: let --signoff override --no-signoff

After resolving a conflicting patch, a user may wish to sign off the
patch to declare that the patch has been modified. As such, the user
will expect that running "git am --signoff --continue" will append the
signoff to the commit message.

However, the --signoff option is only taken into account during the
mail-parsing stage. If the --signoff option is set, then the signoff
will be appended to the commit message. Since the mail-parsing stage
comes before the patch application stage, the --signoff option, if
provided on the command-line when resuming, will have no effect at all.

We cannot move the append_signoff() call to the patch application stage
as the applypatch-msg hook and interactive mode, which run before patch
application, may expect the signoff to be there.

Fix this by taking note if the user explictly set the --signoff option
on the command-line, and append the signoff to the commit message when
resuming if so.

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