--head support for git

Summary:
This adds support for passing a specific HEAD to arc diff, allowing submitting a specific range of commits. This is useful when you want to submit code reviews for past commits without mucking around with the working copy.

This will probably require changes :)

Test Plan: Tested locally, but totally need to add tests for this

Reviewers: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9369