[PATCH in progress] Pristine text store - pristine_install

I have started adjusting the pristine store access functions to
implement the spec in 'notes/wc-ng/pristine-store'. The attached patch
goes most of the way to sorting out the 'install' function, except that
testing for whether it's already installed is still done by looking on
disk for the file whereas it should look for the DB row. That's marked
with a '###' comment.

Other feedback on this will be much appreciated.

- Julian