rebase: introduce --reschedule-failed-exec

A common use case for the `--exec` option is to verify that each commit
in a topic branch compiles cleanly, via `git rebase -x make <base>`.

However, when an `exec` in such a rebase fails, it is not re-scheduled,
which in this instance is not particularly helpful.

Let's offer a flag to reschedule failed `exec` commands.

Based on an idea by Paul Morelle.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sequencer.h

@@ -39,6 +39,7 @@ struct replay_opts {
 	int allow_empty_message;
 	int keep_redundant_commits;
 	int verbose;
+	int reschedule_failed_exec;
 
 	int mainline;