This introduces a new ?files=<id>[,<id>,...] query argument for the

diff viewer URL that allows for specifying specific filediff IDs to

show out of the list of files in that diff. This can be used to

construct versions of the diff viewer that are intended for one team

versus another, or that exclude certain large files.
The new option replaces the old ?file=<id>, which wasn't particularly

functional and may not have worked correctly in the past.  Some rough

support for the old query argument remains.

Unit tests pass.
Manually tested different combinations of IDs on some local diffs. Only

the requested files appeared, but were in the standard sort order.