--range support for git

Summary:
This adds support for passing range of commits for arc diff. 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
changed "--range" to "--head"
added a sanity check that verifies that head is a child of base.