format-patch: teach --range-diff to respect -v/--reroll-count

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

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

revision.h

@@ -222,6 +222,7 @@ struct rev_info {
 	const char *rdiff1;
 	const char *rdiff2;
 	int creation_factor;
+	const char *rdiff_title;
 
 	/* commit counts */
 	int count_left;