Purge loadRelativeEdges

Summary:
Fixes T3821. Maybe. The existing code seemed to have a bug and actually return the //commit phid//. Judging by the function name this is not intended.

Also, sorry to step on toes here -- I thought no one was assigned and was curious about loadRelativeEdges and here we are...

Test Plan: lots of logic here as I have no idea how to use Releeph.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Maniphest Tasks: T3821

Differential Revision: https://secure.phabricator.com/D6967