When loading a diff, the "Download Diff" action would be visible for a

short time, and then vanish. The culprit was that we were setting the

canDownloadDiff attribute in the code that loads a new revision, but

we had optimized the first load a little bit by including the context in

the initial page payload rather than making a separate request.
This change fixes things so that we compute the correct state of this

inside the parse method, which is used for both the initial load as

well as subsequent revisions.

Verified that "Download Diff" was no longer vanishing when it

  shouldn't.
Ran js-tests.