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