Add a "touched paths" limit to repositories, limiting the maximum number of paths any commit may touch

Summary:
Depends on D19831. Ref T13216. See PHI908. Allegedly, a user copied a large repository into itself and then pushed it. Great backup strategy, but it can create headaches for administrators.

Allow a "maximum paths you can touch with one commit" limit to be configured, to make it harder for users to make this push this kind of commit by accident.

If you actually intended to do this, you can work around this by breaking your commit into pieces (or temporarily removing the limit). This isn't a security/policy sort of option, it's just a guard against silly mistakes.

Test Plan: Set limit to 2, tried to push 3 files, got rejected. Raised limit, pushed changes successfully.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13216

Differential Revision: https://secure.phabricator.com/D19839