[zsh] Fix shorten_path

Correctly handle filenames without leading components, and split the
huge globbing monster
[zsh] Fix VCS info in the title

We do not longer need to work around the edge cases of shorten_path
[zsh] Use VCS name and subdir in place of working directory

In a VCS repository, show the name and the repo subdir in place of the
working directory, because that's likely what we really care for.