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.
Add awful.tag.swap
Merge branch 'fix_tag_history_V2' of git://github.com/Elv13/awesome-1
Merge branch 'tag_swap_V2' of git://github.com/Elv13/awesome-1