Worktree 0.16.0

• worktree prune-archives --strip-build-output also strips a Flutter app's build output from an
  existing archive: every section that purely adds an untracked file strictly below <P>/build/
  or <P>/.dart_tool/, where <P> (the tree root included) holds a pubspec.yaml that HEAD's
  tree, read from Git, holds as anything but a directory or that the patch purely adds (that
  section is kept), and where HEAD's tree holds nothing at the directory's own path. A tracked
  source directory named build, and a build/ or .dart_tool/ with no pubspec.yaml beside
  it, are never stripped. The outermost directory whose rule strips the path, cargo target or
  Flutter output, is recorded as a build_output target with origin strip; no manifest field is added and formats 1 to 3 are
  unchanged. archive, finish --archive and discard-cache still recognise only cargo's
  layout. The strip's reasons now say "build layout" rather than "cargo build layout".
• The GitPort::scan_archive_patch port returns a ScannedPatch: the sections with the paths of
  HEAD's files and directories. Marking tracked sections moved from the Git adapter into the
  domain (ScannedPatch::against_head), and plan_patch_strip takes the ScannedPatch.