blackbox: add backup bundle paths to blackbox logs
Writes the backup bundle paths to the blackbox so it's easy to see which
backup bundle is associated with which command when you are debugging an
issue.
Example output:
2013/03/13 10:39:56 durham> strip tip
2013/03/13 10:39:59 durham> saved backup bundle to /data/users/durham/www-hg/.hg/strip-backup/
e5fac262363a-backup.hg
2013/03/13 10:40:03 durham> strip tip exited 0 after 7.97 seconds