R1:dda5c13ef549

Parse "shallow" frames in the Git "upload-pack" wire protocol parser

Summary:
Fixes T13309. If you void the warranty on a repository on disk and turn it into a shallow clone, Phabricator currently can't serve it.

We don't support hosting shallow working copies, but we should still parse and proxy the protocol rather than breaking in a mysterious way.

Test Plan:
- Created a shallow working copy with `mv X X.full; git clone --depth Y file://.../X.full X` in the storage directory on disk.
- Cloned it…