Install a much more fancy git-ps1__ command.

Not very clean... It's a huge hack, but it creates a really useful
prompt (and a really slow one on inadequate hardware). :)
Cleanup dynamic PS1 implementation...

After much deserved berating from #bash.

`-* Export is useless here, you dummy.
`-* Long lines suck (and you can string concat cleanly enough...).
`-* Incremental changes are easier to understand.