Refactor common shell functions out.

Also adds a buildr plugin that can return the absolute path to the checkout root or the subproject root.

For example:
```bash
source "$(buildr -s checkout_root)/bin/bash_functions"
```

Please note that you must invoke buildr with the `-s` option or else you'll get extra output that you don't want.
Ah, that is nice.  It's still nice to have buildr spit out the top directory of the subproject though (since git knows nothing about that).  I will fix the other.