When side-by-side-diffs=1 was set in cgitrc, specifying 'ss=0' in the
querystring would not set the 'unified' option as active in the dropdown
box used to select diffmode.
When side-by-side-diffs=1 was set in cgitrc, specyfing 'ss=0' in the query-
string would not switch to unified diffs. This patch fixes the issue by
introducing a separate variable to track the occurrence of "ss" in the
querystring.
Tim Chen