Use git diff a b for ranges, not git diff a..b

Summary:
These are documented as being identical, but git diff a b works if a is a tree (for example, 4b825d..., the empty tree hash), but git diff a..b does not.

Particularly, with the a..b form, arc diff --base arc:empty does not work. With the a b form, it does.

Test Plan: Ran arc diff --base arc:empty in a repository and got a diff.

Reviewers: btrahan, talshiri

Reviewed By: talshiri

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9898