[exim-dev] Exim git repo maintenance

I've ensured that all the Exim repos on Hummus directly in /srv/git have:

sharedRepository = 0002

A couple of newer repos were missing it. I've also done a chgrp/chmod
so that their content is all in group `eximdev` and has group-write
permission.

I've run the periodic maintenance task:

git fsck && git gc && git repack -Ad

For the Exim repo itself, the `-d` might have been a mistake and made
things slower for those doing a `git pull` the next time. Still, quite
a bit faster for those doing a fresh checkout and not too bad for an
incremental (and after the first incremental after this change, you're
back to normal).

We might consider a cron job to `git repack -A` each main repo on a
monthly basis?

-Phil