Giddie theme updates

* Untracked content has a new symbol: ?
* The branch no longer remains visible when leaving a repo (this was a bug due to `vcs_info` no longer being called).
* Command-mode indication is working.
* Moved the prompt symbol selection into `vcs_info` hooks, which means it should work in backends other than git.
* There is no status indication relating to submodules, and I'm not too bothered about that right now.