add: git smart HTTP daemon

add: git smart HTTP daemon

This removes the need for running `git update-server-info` on the
repositories regularly (or on update), possibly speeds up clones (at
least, git clone can now show progress information), and almost
certainly improves reliability on a pull concurrent with a push to the
same repository (the git daemon can respect lock files, nginx can’t).

(We can also probably remove /srv/git from front-nginx, but I’ll do that
in a separate change.)

Change-Id: Iaf67d8f9f7dbe3b95338d9bb548e7c18bf9f0d0a