- Write over the extra field, marking foo.gz as complete.

   Recovery procedure:
   - If not a replace recovery, read in the foo.add file, and provide that data
     to the appropriate recovery below.  If there is no foo.add file, provide
     a zero data length to the recovery.  In that case, the append recovery
     restores the foo.gz to the previous compressed + uncompressed data state.
     For the the compress recovery, a missing foo.add file results in foo.gz
     being restored to the previous compressed-only data state.
     For the compress recovery, a missing foo.add file results in foo.gz being
     restored to the previous compressed-only data state.
   - Append recovery:
     - Pick up append at + step above
   - Compress recovery:
     - Pick up compress at * step above
   - Replace recovery:
     - Pick up compress at @ step above
   - Log the repair with a date stamp in foo.repairs