Fix paste

Summary:
The caching introduced by rP7e37eb48273eef87e6e6811703fb5d85a3e07a81
broke the use of PhabricatorPaste::getContent() for forking/editing
pastes and downloading pastes with arc paste. I think this fixes all
the appropriate callsites.

Test Plan: fork a paste in the web ui

Reviewers: vrana, epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4244