Don't raise a policy exception if a user can't see the parent revision of a new diff

Summary: Ref T603. We currently bomb out here, but should just continue forward. I'm fairly certain we don't even use this for anything anymore (it has been replaced by "depends on") but need to check that.

Test Plan: Created a new revision with `arc diff`.

Reviewers: ljalonen, btrahan, #blessed_reviewers, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7255