Skip branch-based filtering if the module will use a specific tag instead.

This fixes this warning:

 * Module zanshin is apparently XML-based, but contains no
active modules to build!

when doing this:

module-set
  repository kde-projects
  use-modules zanshin
  tag 0.2.1
end module

kdesrc-build

@@ -910,7 +910,7 @@ sub expandXMLModules
 
         # Bug 307694
         my $moduleSetBranch = $moduleSet->getOption('branch');
-        if ($moduleSetBranch) {
+        if ($moduleSetBranch && !$moduleSet->getOption('tag')) {
             debug ("Filtering kde-projects modules that don't have a $moduleSetBranch branch");
             @xmlResults = grep {
                 list_has($_->{'branches'}, $moduleSetBranch)