awful.tag.update: Fix identical tag set detection

If the "current" table is empty, then identical always true.
There is a lot of case where this can happen. Mostly when using
dynamic tagging.

lib/awful/tag.lua.in

@@ -178,13 +178,15 @@ function tag.history.update(obj)
     else
         if data.history[s].current then
             -- Check that the list is not identical
-            local identical = true
+            local identical = #data.history[s].current == #curtags
+            if identical then
                 for idx, _tag in ipairs(data.history[s].current) do
                     if curtags[idx] ~= _tag then
                         identical = false
                         break
                     end
                 end
+            end
 
             -- Do not update history the table are identical
             if identical then return end