revision: add leak_pending flag

The new flag leak_pending in struct rev_info can be used to prevent
prepare_revision_walk from freeing the list of pending objects.  It
will still forget about them, so it really is leaked.  This behaviour
may look weird at first, but it can be useful if the pointer to the
list is saved before calling prepare_revision_walk.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.c

@@ -1974,7 +1974,8 @@ int prepare_revision_walk(struct rev_info *revs)
 		}
 		e++;
 	}
-	free(list);
+	if (!revs->leak_pending)
+		free(list);
 
 	if (revs->no_walk)
 		return 0;

revision.h

@@ -97,6 +97,7 @@ struct rev_info {
 			date_mode_explicit:1,
 			preserve_subject:1;
 	unsigned int	disable_stdin:1;
+	unsigned int	leak_pending:1;
 
 	enum date_mode date_mode;