in 0437adc33a, we stop right before
reaching $top_srcdir, but we should stop at its parent directory.
in this change, instead of trying to be smart and to walk all the way
up to the root directory or $top_srcdir, we just check $top_srcdir/.git
directly, as we just know it's there or it does not exist at all.
Signed-off-by: Kefu Chai <kchai@redhat.com>