Use --no-ext-diff in arc land call to git diff

Summary:
Fixes T11435. This isn't a perfect solution since there's a little code duplication, but a perfect solution is probably a bit more involved.

See T11435 for some discussion. In particular, most git diff commands already get this flag via ArcanistGitAPI->getDiffBaseOptions().

Test Plan: Will land this change.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11435

Differential Revision: https://secure.phabricator.com/D16375