DB lock file, representing locks on the versioned filesystem.

All accessors -- both readers and writers -- of the repository's
Berkeley DB environment take out shared locks on this file, and
each accessor removes its lock when done.  If and when the DB
recovery procedure is run, the recovery code takes out an
exclusive lock on this file, so we can be sure no one else is
using the DB during the recovery.

You should never have to edit or remove this file.