format-patch: teach --interdiff to respect -v/--reroll-count

The --interdiff option introduces the embedded interdiff generically as
"Interdiff:", however, we can do better when --reroll-count is specified
by emitting "Interdiff against v{n}:" instead.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.h

@@ -215,6 +215,7 @@ struct rev_info {
 	/* interdiff */
 	const struct object_id *idiff_oid1;
 	const struct object_id *idiff_oid2;
+	const char *idiff_title;
 
 	/* commit counts */
 	int count_left;