Speed up __git_ps1 with many untracked files

Note: This isn't intended as a serious fix to the issue, but it does work.

If you use __git_ps1, have GIT_PS1_SHOWUNTRACKEDFILES enabled, and have, say, millions of untracked files in your repository, then you have probably discovered that your shell prompt can become unusably slow.

This is because git ls-files is being run on the repository by __git_ps1, and this takes a very long time when there are a lot of untracked files. However, all we want to know is whether there are any untracked files. And git status runs very quickly still, so let's just use that...