tether 0.1.0b2

Added

• tether drop BOOKMARK (Repo.plan_drop / apply_drop / drop): throwing
  a bookmark away is one command, the opposite of promote. In order: leave
  the bookmark when this checkout is on it (--to, default the trunk), drop
  the commits only it reaches (jj: jj abandon; git: nothing reaches them
  once the branch is gone), delete it, then the store side -- gc --prune-bookmarks restricted to its branches, the pins nothing references
  once the commits are gone, and with --delete-stores the experimental
  created-store step. One rule is drop's own: a branch whose head a dropped
  commit pinned or recorded is deleted (committed work thrown away by name),
  where gc keeps it as "unpinned writes"; uncommitted writes still need
  --force-prune. Dry-run by default; the plan previews the store side as it
  will be once the commits are gone and apply re-plans it live; the set of
  commits only the bookmark reaches is re-derived at apply and a saved plan is
  refused if another bookmark has come to reach one of them. "Only the
  bookmark reaches" counts every reacher the VCS knows -- other bookmarks,
  tags, remote bookmarks, other workspaces' working copies -- and the plan
  notes a feature@origin that still reaches the line. Refused for the
  trunk, for a bookmark another live checkout works on, and when --to
  names one; not undoable by tether (the CLI guide gives the three-step
  recovery). New plan verbs leave-bookmark, abandon-commit,
  delete-bookmark; precondition bookmark_head; GcScope for
  plan_gc(scope=); VcsAdapter.exclusive_commits / remote_counterparts
  / files_at_many / drop_bookmark. The use-cases story's three
  retirements are one line each now. abandon REV [--gc] stays as the
  surgical form: commits off a bookmark you keep.

Fixed

• keep-store is an informational plan action: a gc plan holding only
  keep-store lines is empty, like one holding only keep-branch.