Don't show changes for commits which affect more than 1,000 files

Summary: @nh, does this do something reasonable on merges? We can refine the behavior ('click to show all 92 million files'), but I want to make sure it's at least feasible before we pursue it.

Test Plan: Set 1000 to "3" and looked at a change which touched 6 files.

Reviewers: nh, vrana, zjwsoft

Reviewed By: nh

CC: aran

Differential Revision: https://secure.phabricator.com/D4730