A constant stream of traffic to either httpd/nntpd would mean
git-cat-file processes never expire. Things can go bad after a
full repack, as a full repack will unlink old pack indices and
git-cat-file does not currently detect unlinked files.
We could do something complicated by recursively stat-ing
objects/pack of every git directory and alternate;
but that's probably not worth the trouble compared to
occasionally restarting the cat-file process.
So simplify the code and let httpd/nntpd expire them
periodically, since spawning a "git-cat-file --batch" process
isn't too expensive. We already spawn for every request which
hits git-http-backend, cgit, and git-apply.
In the future, we may optionally support the Git::Raw module
to avoid IPC; but we must remain careful to not leave lingering
FDs open to unlinked files after repack.