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>
Use CatPipe, not show, in git_commit_dates()

Import the commit parser from the experimental bup-get branch and use
it along with CatPipe to produce the git commit dates in
git_commit_dates().

This appears to resolve the performance problem with real archives
that was introduced by the use of "git show -s --pretty=format:%ct
..." (cf. 00ba9fb811e71bb6182b9379461bc6b493e3b7a4), and is still much
faster than 0.25 for at least a 1000 branch synthetic repository here.

Thanks to Gabriel Filion <gabster@lelutin.ca> for reporting the
problem, and to him and Patrick Rouleau <prouleau72@gmail.com> for
helping test the solution.

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