Move VFS cp() to git.py; handle repodir changes

Move VFS cp() to git.py; handle repodir changes

Move the VFS's cp() to git.py and reset the cp() CatPipe whenever the
global repodir changes.

Previously, if two different lib/bup/t/ tests (for example) needed to
use two different repositories, and both happened to indirectly call
cp(), the second test would end up with a CatPipe() connected to the
wrong repository.

In the longer run, we may want to consider further cleanup here, but
this should fix the immediate problem without too much risk.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>