playground refactoring: dgit: Use playground facilities

playground refactoring: dgit: Use playground facilities

* Replace implementation of prep_ud with a call to fresh_playground
* Replace references to $ud with $playground
* Replace hardcoded changedir ../../.. etc. with $maindir

The overall functional change is fairly minimal.  We now call getcwd
more often, and when we want to return to the maindir we changedir to
an absolute path instead of a relative one.

Later, we may rename prep_ud, mktree_in_ud_here, etc.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>