R1:75e89c0c7848

Amended D2009 based on feedback from @epriestley.

Test Plan:
Try out https://secure.phabricator.com/maniphest/view/projectall/?g=j with tasks assigned to just one project,
and also with tasks assigned to more than one project.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D2012