build: correct sed transformation to work also on .git-module-status

build: correct sed transformation to work also on .git-module-status

My patch was wrong.
I kept the sed transformation the same in the two places,
but it wasn't strict enough to also work on the file contents.
Sorry about that.  This fixes it.

>From 93927cc7b9f63c414e5bfeb7eba393fde3295601 Mon Sep 17 00:00:00 2001
From: Jim Meyering <meyering@redhat.com>
Date: Fri, 20 Nov 2009 16:23:17 +0100
Subject: [PATCH libguestfs] build: correct sed transformation to work also on .git-module-status

* autogen.sh: Use a more strict sed transformation so it works also
on the contents of .git-module-status, which has no prefix.
* cfg.mk (_submodule_hash): Use a stricter sed regexp.