R1:45e93495e409

Add method for loading relative edges

Summary:
More and more relations are going under edges and I can't work with them from Relatives framework.

This doesn't have the nice transitive property of normal relatives (loading relative objects from relatives loads all of them at once) but I can add it when I need it.

I plan to use it in D3085 (after converting relationships to edges).

Test Plan:
$task = id(new ManiphestTask())
->loadOneWhere('phid = %s', $phid);
print_r($task->loadRelativeEdges(4))…