--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: epriestley, Blessed Reviewers
Reviewed By: epriestley, Blessed Reviewers
Subscribers: epriestley, Korvin
Differential Revision: https://secure.phabricator.com/D9369