build: Silence git warning under podwrapper

build: Silence git warning under podwrapper

If the current git checkout has configured an orderfile for saner
reading of diffs, this configuration bleeds through to the use of git
within podwrapper for annoying warnings like:

perl /home/eblake/libnbd/podwrapper.pl --section=3 --man nbd_create.3 \
    --html ../html/nbd_create.3.html \
    nbd_create.pod
fatal: failed to read orderfile 'scripts/git.orderfile': No such file or directory

Although the warnings have no impact on the build, we might as well
silence them (since the use of the orderfile is worthwhile).

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