awful.util: Make 'util.table.merge' return the table

This avoid having to add a temporary variable when returning
from functions. Qt does the same.

lib/awful/util.lua

@@ -571,10 +571,12 @@ end
 --- Merge items from the one table to another one
 -- @tparam table t the container table
 -- @tparam table set the mixin table
+-- @treturn table Return `t` for convenience
 function util.table.merge(t, set)
     for _, v in ipairs(set) do
         table.insert(t, v)
     end
+    return t
 end