object: rename function 'typename' to 'type_name'

Rename C++ keyword in order to bring the codebase closer to being able
to be compiled with a C++ compiler.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/diff-tree.c

@@ -76,7 +76,7 @@ static int diff_tree_stdin(char *line)
 	if (obj->type == OBJ_TREE)
 		return stdin_diff_trees((struct tree *)obj, p);
 	error("Object %s is a %s, not a commit or tree",
-	      oid_to_hex(&oid), typename(obj->type));
+	      oid_to_hex(&oid), type_name(obj->type));
 	return -1;
 }