tag: Fix index calculation.

The index was updated on an unordered table. As the elements
order did not match the relative indices once they have been
changed, further calls to set_index produced garbage.

The default taglist didn't notice because it use screen.tags
table index instead of the tag index. A debug using

echo 'for _,t in ipairs(mouse.screen.tags) do
  print("INDEX:", _, t.index, t.name) end' | awesome-client

Would have shown two or more elements with the same index. To
debug issues related to tag indices, this bash script can be
enabled:

while true; do
  echo 'for _,t in ipairs(mouse.screen.tags) do
  assert( _==t.index) end' | awesome-client
  sleep 0.5
done

lib/awful/tag.lua

@@ -74,6 +74,12 @@ function tag.object.set_index(self, idx)
     -- screen.tags cannot be used as it depend on index
     local tmp_tags = raw_tags(scr)
 
+    -- sort the tags by index
+    table.sort(tmp_tags, function(a, b)
+        local ia, ib = tag.getproperty(a, "index"), tag.getproperty(b, "index")
+        return (ia or math.huge) < (ib or math.huge)
+    end)
+
     if (not idx) or (idx < 1) or (idx > #tmp_tags) then
         return
     end
@@ -97,13 +103,14 @@ function tag.object.set_index(self, idx)
 end
 
 function tag.object.get_index(query_tag)
-    -- Get an unordered list of tags
-    local tags = raw_tags(query_tag.screen)
 
     local idx = tag.getproperty(query_tag, "index")
 
     if idx then return idx end
 
+    -- Get an unordered list of tags
+    local tags = raw_tags(query_tag.screen)
+
     -- Too bad, lets compute it
     for i, t in ipairs(tags) do
         if t == query_tag then