blackbox: store the blackbox ui object instead of the log file
Without this, the last logged entry didn't have access to
the repository, and thus couldn't report its version
(and especially that an add or similar dirtied it).
A side-effect is that one repo leaks until process exit...
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present