cmd/restore: embarrassingly slow implementation of 'bup restore'

cmd/restore: embarrassingly slow implementation of 'bup restore'

Well, that was easy, since vfs.py already existed and is doing most of the
hard work.  Only 103 lines including all the log message handling and
whatnot.

Only one catch: the restoring code is definitely not optimized.  Among other
things (like the probably-excessive-for-our-restoring-needs layering in
vfs.py), we're still calling into 'git cat-file --stdin' to retrieve our
objects.  This involves lots and lots of context switches, plus it can't use
midx files for its lookups.  The result is that restoring takes much more
CPU time and memory than it really should.  But oh well, we have to start
somewhere.

Signed-off-by: Avery Pennarun <apenwarr@gmail.com>