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