Worktree 0.19.0

• A .worktreekeep in a subdirectory keeps paths at and below that directory, its patterns matched
  relative to it as Git reads a nested .gitignore. discard-cache, sweep and finish --discard-cache --archive keep what it names, archive images it, and prune-archives --strip-build-output never strips it. It keeps nothing outside its own directory, is never
  deleted itself, and one that is a symbolic link, a directory or unreadable refuses as
  keep-file-invalid like the root file.

• A nested .worktreekeep only adds keeps: its ! lines are ignored, so it cannot un-keep what the
  root file or its own other lines keep, and a nested file keeps a path the root file negates. The
  .gitignore of a directory that holds a nested .worktreekeep is kept with it by discard-cache,
  sweep, finish --discard-cache and the strip, so a tool's self-ignoring directory still hides
  what the keep file keeps.