git-changed-in-head now always returns 0

A change deleting a single PHP file, would make git show to return an
empty list.  The grep command would thus fail to find a newline '^$' and
exit 1 which breaks the Jenkins job.

By using `|| :`, we make sure the exit code will be 0 and the Jenkins
job can continue its processing.

Change-Id: Ib31e89fcaf59df6f8d01f4e785576247dd6cae42