revision: convert to using diff_tree_sha1()

Since diff_tree_sha1() can now accept empty trees via NULL sha1, we
could just call it without manually reading trees into tree_desc and
duplicating code.

Besides, that

	if (!tree)
		return 0;

looked suspect - we were saying an invalid tree != empty tree, but maybe it is
better to just say the tree is invalid here, which is what diff_tree_sha1()
does for such case.

Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

revision.c

@@ -496,24 +496,14 @@ static int rev_compare_tree(struct rev_info *revs,
 static int rev_same_tree_as_empty(struct rev_info *revs, struct commit *commit)
 {
 	int retval;
-	void *tree;
-	unsigned long size;
-	struct tree_desc empty, real;
 	struct tree *t1 = commit->tree;
 
 	if (!t1)
 		return 0;
 
-	tree = read_object_with_reference(t1->object.sha1, tree_type, &size, NULL);
-	if (!tree)
-		return 0;
-	init_tree_desc(&real, tree, size);
-	init_tree_desc(&empty, "", 0);
-
 	tree_difference = REV_TREE_SAME;
 	DIFF_OPT_CLR(&revs->pruning, HAS_CHANGES);
-	retval = diff_tree(&empty, &real, "", &revs->pruning);
-	free(tree);
+	retval = diff_tree_sha1(NULL, t1->object.sha1, "", &revs->pruning);
 
 	return retval >= 0 && (tree_difference == REV_TREE_SAME);
 }