make arc diff link maniphest tasks with revisions

Summary:
add "Maniphest Task:" or "Maniphest Tasks:" followed by text that has TX in it.
foreach TX the task will be attached to the revision and the revision will be
attached to the task. parsing is pretty... ummm, robust such that it will pick
up any TX substring and parse that as a Maniphest Task just fine. it errors
out if there is not an actual task for TX and otherwise churns along pretty
nicely.

Also, make sure the PhabricatorObjectHandle loads the task ID as the alternateID
since we need that here and it should be that way anyhoo.

Test Plan:
made a diff and in the commit message added Maniphest Task(s): TX combination.
Tried various combinations of TX -- single, multiple with commas, multiple many
lines, single bad, multiple bad, multiple mix of bad and good. verified that the
good tasks were attached to the diff and diff was attached to the good tasks.

Maniphest Tasks: T137

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, btrahan, epriestley

Differential Revision: 1165