merge-recursive.c: mrtree in merge() is not used before set

The called function merge_trees() sets its *result, to which the
address of the variable mrtree in merge() function is passed,
only when index_only is set.  But that is Ok as the function
uses the value in the variable only under index_only iteration.

However, recent gcc does not realize this.  Work it around by
adding a fake initializer.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-recursive.c

@@ -1586,7 +1586,7 @@ static int merge(struct commit *h1,
 {
 	struct commit_list *iter;
 	struct commit *merged_common_ancestors;
-	struct tree *mrtree;
+	struct tree *mrtree = mrtree;
 	int clean;
 
 	if (show(4)) {