When a diff has 12,345 lines, render "12,345 lines" instead of "12 lines"

Summary: This `%d` should be a `%s`, since the `PhutilNumber` value may get formatted according to locale settings.

Test Plan: will make @zeeg

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley, zeeg

Differential Revision: https://secure.phabricator.com/D8814