* Update various comments, include expected arguments and example prompt.
* Use IFS to split GIT_REPO_INFO on \n rather than spaces.
* If the repository has no commits yet, display "!NO COMMITS!" as the branch.
* Use the GIT_REPO_INFO for top-directory instead of another call out to git.
* Use pwd -P to force actual paths not following symlinks.
* Pull upstream status if GIT_PROMPT_AUTO_PREFETCH is set, the current
directory is the git repo top-dir and maintenance tasks for this repo have
been enabled. This allows reporting whether the branch is behind the
remote by any number.