R1:54154e4f48d0

Move "Rendering References" to the DifferentialChangesetParser level

Summary:
Separates changeset IDs from rendering. Now each changeset has a "rendering
reference" which is basically a description of what the ajax endpoint should
render. For Differential, it's in the form "id/vs". For Diffusion,
"branch/path;commit".

I believe this fixes pretty much all of the bugs related to "show more" breaking
in various obscure ways, although I never got a great repro for T153.

Test Plan:
Clicked "show more" in…