The "/fdiff" page:

URL: fdiff?v1=UUID&v2=UUID

Two arguments, v1 and v2, identify the artifacts to be diffed.
Show diff side by side unless sbs is 0.  Generate plain text if
"patch" is present, otherwise generate "pretty" HTML.

Alternative URL:  fdiff?from=filename1&to=filename2&ci=checkin

If the "from" and "to" query parameters are both present, then they are
the names of two files within the check-in "ci" that are diffed.  If the
"ci" parameter is omitted, then the most recent check-in ("tip") is
used.

Additional parameters:

     dc=N         Show N lines of context around each diff
     patch        Use the patch diff format
     regex=REGEX  Only show differences that match REGEX
     sbs=BOOLEAN  Turn side-by-side diffs on and off (default: on)
     verbose      Show more detail when describing artifacts
     w            Ignore whitespace