Make Phabricator clone bare git repositories

Summary:
This doesn't really impact anything very much, but is a little cleaner than cloning repositories with a working copy. It's somewhat important for allowing pushes, because you can't push to a checked-out branch.

Mercurial has a similar option (--noupdate) but leave that alone for now.

The origin stuff was mostly for sanity/explicitness purposes -- I believe it's safe to remove in all non-ridiculous cases. Git fails with it in bare repositories (it automatically creates an origin, but doesn't create the local refs for it, or something).

Test Plan: Nuked a repo, re-cloned it, pulled and updated it several times. Browsed both bare and non-bare repos in Diffusion.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2230

Differential Revision: https://secure.phabricator.com/D7430