FF: changes to versionChooser to work with new psychopy/releases repos

also a few other thigns cleaned up a little:

1. a class for temporary chdir so that we can skip the try...finally
2. check if there's a local copy of that tag already (don't always
fetch)
3. check if we already have the version checked out (don't need to
checkout)
4. slightly different path for the local repos