viewdiff: add "b=" param with non-standard diff prefix

viewdiff: add "b=" param with non-standard diff prefix

<20180228012207.GB251290@aiede.svl.corp.google.com> (posted to
git@vger) uses "i" and "w" prefixes instead of the standard "a"
and "b" prefixes, ensure we emit a "b=$FILENAME" param for the
solver endpoint to improve search accuracy, syntax highlighting,
and information density in the URL itself.