rebase: add a config option to default to --reschedule-failed-exec

It would be cumbersome to type out that option all the time, so let's
offer the convenience of a config setting: rebase.rescheduleFailedExec.

Besides, this opens the door to changing the default in a future version
of Git: it does make some sense to reschedule failed `exec` commands by
default (and if we could go back in time when the `exec` command was
invented, we probably would change that default right from the start).

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

builtin/rebase.c

@@ -677,6 +677,11 @@ static int rebase_config(const char *var, const char *value, void *data)
 		return 0;
 	}
 
+	if (!strcmp(var, "rebase.reschedulefailedexec")) {
+		opts->reschedule_failed_exec = git_config_bool(var, value);
+		return 0;
+	}
+
 	return git_default_config(var, value, data);
 }