Git cl decides if git-numberer is enabled on a repository by writing
Gerrit's project.config from refs/meta/config into a tempfile, which is
then queried using `git config -f tempfile --get ...`.  The file itself
is only flushed, but not closed after writing because Python's
tempfile.NamedTemporaryFile is deleted on closing. This worked fine on
Linix/Mac, but not on Windows, where `git config` apparently doesn't see
file or its contents.

This CL rewrites the above using yet another contexmanager temp
directory into which a file is written and closed before git config is
ran.

R=machenbach@chromium.org,grt@chromium.org
BUG=683202

Change-Id: I7974d66b1b2b0478ab4b6f7ac04e547a4981c46c
Reviewed-on: https://chromium-review.googlesource.com/430719
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>