rebase -i: support --ignore-date

As part of the on-going effort to retire the apply rebase backend
teach the merge backend how to handle --ignore-date. We take care to
handle the combination of --ignore-date and
--committer-date-is-author-date in the same way as the apply backend.

Original-patch-by: Rohit Ashiwal <rohit.ashiwal265@gmail.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.h

@@ -46,6 +46,7 @@ struct replay_opts {
 	int quiet;
 	int reschedule_failed_exec;
 	int committer_date_is_author_date;
+	int ignore_date;
 
 	int mainline;