merge-recursive: take advantage of hardcoded empty tree

When this code was first written (v1.4.3-rc1~174^2~4, merge-recur: if
there is no common ancestor, fake empty one, 2006-08-09), everyone
needing a fake empty tree had to make her own, but ever since
v1.5.5-rc0~180^2~1 (2008-02-13), the object lookup machinery provides
a ready-made one.  Use it.

This is just a simplification, though it also fixes a small leak
(since the tree in the virtual common ancestor commit is never freed).

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-recursive.c

@@ -1309,12 +1309,10 @@ int merge_recursive(struct merge_options *o,
 
 	merged_common_ancestors = pop_commit(&ca);
 	if (merged_common_ancestors == NULL) {
-		/* if there is no common ancestor, make an empty tree */
-		struct tree *tree = xcalloc(1, sizeof(struct tree));
+		/* if there is no common ancestor, use an empty tree */
+		struct tree *tree;
 
-		tree->object.parsed = 1;
-		tree->object.type = OBJ_TREE;
-		pretend_sha1_file(NULL, 0, OBJ_TREE, tree->object.sha1);
+		tree = lookup_tree((const unsigned char *)EMPTY_TREE_SHA1_BIN);
 		merged_common_ancestors = make_virtual_commit(tree, "ancestor");
 	}