v6.8.0

Fixes

• Config files are written atomically. A crash or full disk during -w, -W,
  or --write-config-file can no longer truncate a hand-written config.

Features

• Timestamps survive picopt upgrades that record new options. Only a changed
  option value invalidates them, so stamp files written before an option joined
  the recorded set stay valid at its default. Timestamps from before 6.7.0 work
  again.
• Timestamp files now open with a comment explaining what they are, that they
  are machine-written, and that deleting one just re-optimizes that tree.
• Discarding timestamps because a sub-directory .picopt.yaml changed now says
  so, instead of naming an internal key.
• README documents .picopt_treestamps.yaml: what it records, that it is never
  applied as configuration, and that it is safe to delete.

Changes

• Requires treestamps 5.0.0. Timestamp files no longer duplicate ignore and
  symlinks in a second block. Existing timestamp files still load, and
  upgrading invalidates nothing.