build: fix 'make check' with newer git

build: fix 'make check' with newer git

Newer git doesn't like the maint.mk rule 'public-submodule-commit'
run during 'make check', as inherited from our checkout of gnulib.
I tracked down that libvirt commit 8531301 picked up a gnulib fix
that makes git happy.  Rather than try and do a full .gnulib
submodule update to gnulib.git d18d1b802 (as used in that libvirt
commit), it was easier to just backport the fixed maint.mk from
gnulib on top of our existing submodule level.  I did it as follows,
where these steps will have to be repeated when cherry-picking this
commit to any other maintenance branch:

mkdir -p gnulib/local/top
cd .gnulib
git checkout d18d1b802 top/maint.mk
git diff HEAD > ../gnulib/local/top/maint.mk.diff
git reset --hard
cd ..
git add gnulib/local/top

Signed-off-by: Eric Blake <eblake@redhat.com>