DiffReviewable has a private function, _buildRenderedDiffURL(), for

generating the URL for a rendered diff, which constructs the URL based

on some instance state.  There's then a couple of functions that use

that and try to modify it to suit their needs.
This falls apart when a query string is returned. Some code assumes it

can append a path to that, which fails in the case where someone's

trying to expand code in the diff viewer while looking at a commit with

a base commit ID.
Now, _buildRenderedDiffURL() takes in all the parameters needed to

construct the URL, and does so correctly. Callers only need to pass in

what they need, and never need to modify the result.
The URL also always contains the TEMPLATE_SERIAL now in the query

string, to ensure proper cache busting (and to further discourage

appending paths to the URL).
This should ensure we always have reliable, tested URLs going forward.

Tested rendering and expanding chunks in:

Normal diffs
Interdiffs
Commits/ranges
Commits with interdiffs