rev-list --count: separate count for --cherry-mark

When --count is used with --cherry-mark, omit the patch equivalent
commits from the count for left and right commits and print the count of
equivalent commits separately.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.h

@@ -141,6 +141,7 @@ struct rev_info {
 	/* commit counts */
 	int count_left;
 	int count_right;
+	int count_same;
 };
 
 #define REV_TREE_SAME		0