Fix `differential.getdiff`, etc., for diffs with no Arcanist Project

Summary:
`getArcanistProjectName()` has some logic which gets messy with the `self::ATTACHABLE` mechanism. This makes `differential.getdiff` and similar Conduit methods throw an exception when querying a diff which doesn't have a project. See <http://pastebin.com/Czzrd0Jz>.

Instead, unconditionally attach a project (possibly `null`) when loading diffs if they need projects.

Test Plan: Ran `differential.getdiff` against a `arc diff --raw` diff with no project, got a result instead of an exception.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, sttwister

Differential Revision: https://secure.phabricator.com/D7101