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 diffusion change and commit views and differential diff,
diff-of-diff, standalone-diff, standalone-diff-of-diff views. Verified refs and
'whitespace' were always sent correctly.

Made inline comments on diffs and diffs-of-diffs. Used "Reply".

Reviewed By: tuomaspelkonen
Reviewers: tuomaspelkonen, jungejason, aran
CC: aran, tuomaspelkonen, epriestley
Differential Revision: 274

src/applications/differential/controller/changesetview/DifferentialChangesetViewController.php

@@ -24,9 +24,17 @@ class DifferentialChangesetViewController extends DifferentialController {
 
     $author_phid = $request->getUser()->getPHID();
 
-    $id = $request->getStr('id');
-    $vs = $request->getInt('vs');
+    $rendering_reference = $request->getStr('ref');
+    $parts = explode('/', $rendering_reference);
+    if (count($parts) == 2) {
+      list($id, $vs) = $parts;
+    } else {
+      $id = $parts[0];
+      $vs = 0;
+    }
 
+    $id = (int)$id;
+    $vs = (int)$vs;
 
     $changeset = id(new DifferentialChangeset())->load($id);
     if (!$changeset) {
@@ -149,6 +157,7 @@ class DifferentialChangesetViewController extends DifferentialController {
 
     $parser = new DifferentialChangesetParser();
     $parser->setChangeset($changeset);
+    $parser->setRenderingReference($rendering_reference);
     $parser->setRenderCacheKey($render_cache_key);
     $parser->setRightSideCommentMapping($right_source, $right_new);
     $parser->setLeftSideCommentMapping($left_source, $left_new);