Fix --depth N clones in Git

Summary: Ref T2230. Fixes T4079. As it turns out, this is Git being weird. See comments for some detials about what's going on here.

Test Plan: Created shallow and deep Git clones.

Reviewers: hach-que, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4079, T2230

Differential Revision: https://secure.phabricator.com/D7554