Include untracked files in dirtiness check

Things to consider:

1. should we remove the `# fastest possible way to check if repo is dirty` comment, because it's not *the* ultimate fastest solution, it's kind of a comparable performance to few others, i guess;

2. on my machine both this and the original solution (i.e. `git diff --quiet --ignore-submodules HEAD &>/dev/null`) still takes quite a bit of time to get me to the prompt when i'm in the WebKit repo (from about 1-2s up to 9-10s). I'm not working on such a big codebases, but if i were i would like to be able to disable the dirtiness check entirely for a particular session, because it really slows you down if you have to use terminal a bit inside the repo. Should we hide the entire dirtiness check under a flag as well, so people could just set it to false when they need to?

#1, #79
Yes, but make it on by default.