Allow phabricator to manage repositories

Summary:
This adds a configuration option for repositories to be marked as managed
by phabricator, which is then used by the pullLocal daemon to try to recover
from some errors it runs into.

Test Plan: Set a bogus uri for a repo, saw that the pullLocal daemon fixed it.

Reviewers: epriestley, vrana

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3680