viewdiff: add "b=" param when missing "diff --git" line

viewdiff: add "b=" param when missing "diff --git" line

<2841d2de-32ad-eae8-6039-9251a40bb00e@tngtech.com> as posted to
git@vger contained an otherwise valid diff without a "diff
--git" line.  Generate a "b=" parameter in that case using the
"+++" line instead of the "diff --git" line.  SearchIdx.pm no
longer uses the "diff --git" line for filename information,
either.