Run a commit discovery daemon only once

Summary:
Discover commits then return; useful when initializing new repositories
in unit tests.

By which I mean "when initializing a new repository in my unit test that
I'm working on".

Test Plan: Using this in a PhabricatorTestCase.

Reviewers: epriestley, aran

Reviewed By: epriestley

CC: aran, edward, epriestley

Differential Revision: 948