Search owners by repository

Summary: I've chosen passing callsign even if it is more complicated because it
is nicer than PHID and can be written by hand.

Test Plan:
Search without repository.
Search with repository.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1571