Add git alias for tags.

"tg" is for showing a list of tags with the messages and "tgg" is for showing
the tag, messages, and commit info.
Add "purge" to git completion bash. How cool is that?

Basically, all you have to do to allow a custom utility autocomplete is add
your command in the case statement around line 2100 with the completion type it
is most similar to, i.e. _git_stach, _git_log, _git_mv, etc.