tether 0.1.0b6

Added

• tether init --dataset-id ID uses that id instead of a fresh one; anything
  but 8 lowercase hex characters is refused.
• tether restore KEY... --at REF starts the bookmark's branch as a copy of
  a native branch, tag or state id. REF itself is never moved.
• tether new --adopt takes the bookmark's existing store branches as they
  are, uncommitted writes included, instead of resetting them. A saved plan
  binds to each branch still existing and being the newest generation, not
  to its head.
• tether recover lists tether's refs in each store by dataset id and prints
  the steps that take back a dataset whose repository was lost.
• tether repair KEY... (Repo.repair(keys)) rebuilds only those objects'
  pins and branches, selected from every key history has had with
  --all-history. A saved plan records its selection under its digest.

Changed

• A saved plan (--plan FILE) records the tether version that made it, and
  --from-plan applies it only under that version. Plans saved by any other
  version, including every earlier release, are refused: re-run the plan.
• tether recover prints each step as a command followed by a # note, and
  --json gives each as {"command": str or null, "note": str} instead of
  one string mixing the two.

Fixed

• tether new --shared can join a bookmark whose branch is being written,
  such as a preview environment's database. The plan binds to the branch
  still building on the bookmark's pin, not to its head, which moved on.
• tether new BOOKMARK (or new REV) to a revision that registers an
  object the current checkout lacks no longer fails with "this new plan
  predates the ref_absent/ref_head ... precondition(s)". Its checks now look
  in the stores the target revision names, which differ when an object was
  moved.
• Applying a saved new plan refuses to adopt a branch that has a newer
  generation (.2) or is gone. 0.1.0b5's plans bound an adopt to its head
  only, which a newer generation leaves as it was.
• restore --at and add --at refuse an empty or blank REF. An unset
  variable (--at "$UNSET") used to reset the branch from the upstream head.
  --at 0 (and at=0 from Python) is a version like any other, such as a
  Delta table's first.
• A file or neon object registered with at = 0 is refused like any
  other at, instead of reading the current head.
• tether ops shows a restore --at 0 as restored KEY from 0, not from None.
• tether recover lists per-workspace branches from before bookmarks, which
  may hold uncommitted writes, with a restore --at step for each. It
  suggests no dataset id when run on some keys only, and puts tether repair
  first when a manifest's pin is missing from its store. It also matches
  escaped bookmark names (feature/x) to the VCS's bookmarks.
• tether recover KEY... no longer suggests a tether commit, which pins
  every object under the current id and could lock in the wrong one; its
  last step is tether recover without keys.
• tether recover's commands quote every key, ref, bookmark and message for
  the shell, and its advice for a legacy branch that objects share (two Neon
  databases on one branch) names them in one tether restore, which
  restore requires.
• tether recover KEY... suggests tether repair -- KEY... for only the
  selected objects whose pins are missing, instead of a repair of every
  object that reached the stores the run left out.
• tether recover's legacy-branch steps run from the trunk: each moves to a
  bookmark recover-<workspace> off the trunk first, since restore --at
  refuses the trunk, and commits what it restored.
• tether recover's commands end their options with --, so a key such as
  --help or -x, or a git branch name starting with -, is not read as
  an option.
• tether recover joins an existing recover-<workspace> bookmark only when
  it is an earlier recovery's (nothing but its commits, and the objects
  registered); an unrelated bookmark of that name is left alone and the steps
  use recover-<workspace>-2.
• git: a branch whose name starts with - (git update-ref makes one) is
  read as a name by every git call tether makes, so tether new -- -feat
  switches to it instead of failing to resolve it.