blackbox: do not prevent 'chg init' from working
Previously, blackbox always appends to blackbox.log and creates the
directory for that file on demand. That could be an issue if:
1. chg starts from `$REPO` directory, so `ui._bbrepo` is set.
2. `rm -rf $REPO`.
3. `chg init $REPO`, blackbox writes something and `init` will fail
because `$REPO` directory is non-empty.
This patch fixes that by verifying whether vfs exists before re-using it.
Differential Revision: https://phab.mercurial-scm.org/D768
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.