Fix two issues with callsign-free repositories

Summary:
Ref T4245. These callsites don't quite do the right thing if a repository has no callsign.

See also https://github.com/phacility/phabricator/pull/822.

Test Plan: Made a comment on a commit in a repository with no callsign.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D15344