Start adding interfaces to read in partial trees

The same way "git-diff-tree" can limit its output to just a set of matches,
we can read in just a partial tree for comparison purposes.

read-tree.c

@@ -17,7 +17,7 @@ static int unpack_tree(unsigned char *sha1)
 	buffer = read_object_with_reference(sha1, "tree", &size, NULL);
 	if (!buffer)
 		return -1;
-	ret = read_tree(buffer, size, stage);
+	ret = read_tree(buffer, size, stage, NULL);
 	free(buffer);
 	return ret;
 }