Show "show more..." link in "Local Commits" view in Differential

Summary: Javascript! Depends on D2437 to do anything useful.

Test Plan: Clicked "show more", saw more.

Reviewers: csilvers, btrahan

Reviewed By: csilvers

CC: aran

Maniphest Tasks: T1189

Differential Revision: https://secure.phabricator.com/D2438