phabricator: use parents.set to always set dependencies
Now that Mercurial's Phabricator instance has been updated to a version that
supports the parents.set transaction on revision.edit we can use that to set
dependency relationships in patch stacks instead of abusing the summary.
This has the advantage that we can use it on every phabsend so commit
reordering is picked up without spamming changes like abusing the summary would,
and using parents.set will clear previous parents unlike parents.add.
Differential Revision: https://phab.mercurial-scm.org/D6514