playground refactoring: Dgit.pm: Expose $local_git_cfg

This is now the default for playtree_setup.  The caller can simply set
it explicitly.

We make this part of a new :playground export tag for Dgit.pm,
which is going to gain other bits shortly.

And, we add some comments, and move the global; more code and
commentary will appear around this.

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