replace: make sure --edit results in a different object

It's a bad idea to create a replace ref for an object
that points to the original object itself.

That's why we have to check if the result from editing
the original object is a different object and error out
if it isn't.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/replace.c

@@ -275,6 +275,9 @@ static int edit_and_replace(const char *object_ref, int force)
 
 	free(tmpfile);
 
+	if (!hashcmp(old, new))
+		return error("new object is the same as the old one: '%s'", sha1_to_hex(old));
+
 	return replace_object_sha1(object_ref, old, "replacement", new, force);
 }