merge-recursive: find-renames resets threshold

Make the find-renames option follow the behaviour in git-diff, where it
resets the threshold when none is given. So, for instance,
"--find-renames=25 --find-renames" should result in the default
threshold (50%) instead of 25%.

Signed-off-by: Felipe Gonçalves Assis <felipegassis@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-recursive.c

@@ -2094,8 +2094,10 @@ int parse_merge_opt(struct merge_options *o, const char *s)
 		o->renormalize = 0;
 	else if (!strcmp(s, "no-renames"))
 		o->detect_rename = 0;
-	else if (!strcmp(s, "find-renames"))
+	else if (!strcmp(s, "find-renames")) {
 		o->detect_rename = 1;
+		o->rename_score = 0;
+	}
 	else if (skip_prefix(s, "find-renames=", &arg) ||
 		 skip_prefix(s, "rename-threshold=", &arg)) {
 		if ((o->rename_score = parse_rename_score(&arg)) == -1 || *arg != 0)