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