Allow Commits to be attached to Tasks using edges

Summary: Use Edges to attach Commits and Tasks. Note, no "edit attached commits" interface from tasks yet since the search backend needs a little work to list commits in a sensible way.

Test Plan: Attached commits to tasks. Looked at commits, looked at tasks.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T904

Differential Revision: https://secure.phabricator.com/D2105