Fix arc diff when run from a git submodule

Summary:
A git submodule looks a lot like a normal git repo, but the .git
directory is replaced with a file that git reads to find the real
location of the git directory. When arcanist tries to write a file into
a directory inside of there, it was failing silently, and then crashing
silently when it couldn't read results back out. Instead of assuming the
git directory is a directory named .git at the toplevel of the tree, we
use the appropriate git command to get the correct git directory.

Test Plan: submit a diff from a submodule

Reviewers: epriestley, vrana

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4482