merge-tree: use ll_merge() not xdl_merge()

ll_merge() interface was designed to merge contents under git control
while taking per-path attributes into account.  Update the three-way
merge implementation used by merge-tree to use it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/merge-tree.c b/merge-tree.c
index f01e7c8..37b94d9 100644
--- a/merge-tree.c
+++ b/merge-tree.c
@@ -54,7 +54,7 @@
 	return "removed in remote";
 }
 
-extern void *merge_file(struct blob *, struct blob *, struct blob *, unsigned long *);
+extern void *merge_file(const char *, struct blob *, struct blob *, struct blob *, unsigned long *);
 
 static void *result(struct merge_list *entry, unsigned long *size)
 {
@@ -76,7 +76,7 @@
 	their = NULL;
 	if (entry)
 		their = entry->blob;
-	return merge_file(base, our, their, size);
+	return merge_file(entry->path, base, our, their, size);
 }
 
 static void *origin(struct merge_list *entry, unsigned long *size)