borgmatic 1.5.9

• #300: Add "borgmatic export-tar" action to export an archive to a tar-formatted file or stream.
• #339: Fix for intermittent timing-related test failure of logging function.
• Clarify database documentation about excluding named pipes and character/block devices to prevent
  hangs.
• Add documentation on how to make backups redundant with multiple repositories:
  https://torsion.org/borgmatic/docs/how-to/make-backups-redundant/