Add a diff viewer query argument for showing specific file IDs.

Review Request #9172 — Created Sept. 5, 2017 and discarded — Latest diff uploaded


Review Board


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.