Worktree 0.17.1

• discard-cache (and finish --discard-cache, sweep) never deletes a nested Git repository,
  whatever kind of recognised directory holds it. 0.17.0 deleted a Cargo target, or its tmp/,
  whatever it held, including a fixture repository a test wrote into CARGO_TARGET_TMPDIR with
  commits no remote holds; finish --discard-cache --archive discards before it archives, so it
  lost those commits before the archive could image them.
• The walk 0.17.0 used only for a Flutter app's output now decides for every cache kind: a
  recognised directory holding a .git (a file, a directory or a symbolic link) at any depth
  below it, found by a walk that follows no symbolic link (an entry or directory it cannot read
  counts as one), is never deleted whole. A whole Cargo target holding one is no longer
  cargo-target and falls to its parts, each profile, nested target and tmp/ under its own
  rule. A tmp/ holding one deletes each direct entry holding none (a file or symbolic link
  included) as cargo-target-tmp, reported per entry as <target>/tmp/<entry>, and retains each
  entry holding one. A profile, node_modules, virtual environment, tool cache or Flutter output
  directory holding one is retained whole.
• The walk also recognises a Git directory that has no .git entry, whatever its name: a bare
  repository (such as a test's remote.git in tmp/) or one placed with --separate-git-dir,
  by HEAD as a regular file and objects/ and refs/ as real directories, none reached
  through a symbolic link. Such a directory, or one holding one at any depth, is handled exactly
  as one holding a .git, including as a direct entry of tmp/.
• The walk is repeated immediately before each deletion for every kind, not only Flutter output;
  a directory that is or holds a repository then is retained whole.
• Retained repositories appear in retained_ignored; finish --discard-cache --archive images
  each one, and the tree is still collected through that archive. No cache kind, JSON field or
  protocol version changes.
• Keeping is not recovering. A tree whose kept state the archive cannot image stays, and the
  refusal names the path: a fixture repository with a linked worktree makes
  finish --discard-cache --archive refuse archive-unsupported-entry naming its
  .git/worktrees/<name>, and a directory in tmp/ no walk can read makes gc refuse
  worktree-state-unreadable naming it. Nothing in either is deleted.
• A repository is never build layout in an archive, cargo's or a Flutter app's. 0.17.0 left a
  bare repository inside a profile (target/release/build/<crate>/out/remote.git) out of a
  worktree.archive/3 as cargo build layout, with no image and no blocker; discard-cache kept
  the profile, but gc --apply then removed the tree, and the repository's only copy of its
  commits with it. archive and finish --archive now keep every path with a .git component,
  or at or below a Git directory observed on disk (the same rule discard-cache uses: HEAD a
  regular file, objects/ and refs/ real directories, none reached through a symbolic link), in
  dirty.patch as an ordinary untracked file and in the fingerprint. One I/O-free rule in the
  domain (repository_path, layout_directory) decides it for archive, verification, removal
  and the strip from the observations each supplies (GitDirectoryFacts on disk).
• Verification computes the fingerprint by the same rule, so such an archive verifies against
  its tree, and removal through a format 3 archive re-observes it immediately before deleting a
  left-out file: a path that is now at or below a Git directory, or has a .git component, is
  never deleted as build layout; it must match the archive as any other file does, or refuses.
• prune-archives --strip-build-output never strips a section whose path has a .git
  component, or lies at or below a directory D for which the patch purely adds D/HEAD and at
  least one path under D/objects/ (a Git directory, as the patch can show one). No manifest
  field, format or protocol version changes.