tree-walk.c: remove the_repo from fill_tree_descriptor()

While at there, clean up the_repo usage in builtin/merge-tree.c a tiny
bit.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/tree-walk.c b/tree-walk.c
index 0c7722b..c5569b3 100644
--- a/tree-walk.c
+++ b/tree-walk.c
@@ -81,13 +81,15 @@ int init_tree_desc_gently(struct tree_desc *desc, const void *buffer, unsigned l
 	return result;
 }
 
-void *fill_tree_descriptor(struct tree_desc *desc, const struct object_id *oid)
+void *fill_tree_descriptor(struct repository *r,
+			   struct tree_desc *desc,
+			   const struct object_id *oid)
 {
 	unsigned long size = 0;
 	void *buf = NULL;
 
 	if (oid) {
-		buf = read_object_with_reference(the_repository, oid, tree_type, &size, NULL);
+		buf = read_object_with_reference(r, oid, tree_type, &size, NULL);
 		if (!buf)
 			die("unable to read tree %s", oid_to_hex(oid));
 	}