OK, the 'd3' repository has forced us to become creative with callsigns.

When 'D' is already taken, there's not much left to do.  (I guess we
could do DTHREE but that's cleverer than I want to be at this hour.)
Just add a random letter.

Auditors: alpert

Test Plan:
% python -c "import update_phabricator_repositories as u; print u._create_new_phabricator_callsign('d3', 'git', {'GD'})"
GDA

% python -c "import update_phabricator_repositories as u; print u._create_new_phabricator_callsign('d3', 'git', {'GD', 'GDA'})"
GDB

so it seems to work.