ui-ssdiff.c: set correct diffmode in "control panel"

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.

ui-diff.c

@@ -341,9 +341,7 @@ void cgit_print_diff_ctrls()
 	html("<td class='label'>mode:</td>");
 	html("<td class='ctrl'>");
 	html("<select name='ss' onchange='this.form.submit();'>");
-	curr = ctx.qry.ssdiff;
-	if (!curr && ctx.cfg.ssdiff)
-		curr = 1;
+	curr = ctx.qry.has_ssdiff ? ctx.qry.ssdiff : ctx.cfg.ssdiff;
 	html_intoption(0, "unified", curr);
 	html_intoption(1, "ssdiff", curr);
 	html("</select></td></tr>");