Show a worktree as the branch it holds, in one place in the sidebar - #710

A worktree is somewhere work happens, so the group sits with the
branches rather than down among the remotes and the vendored things.

- Add a WORKTREES group listing every worktree git reports, filled at
  -populateList and rebuilt when a later snapshot arrives, the way the
  stashes group already is.
- Mark the worktree this window has open with the checkmark that means
  exactly that elsewhere in the sidebar, and give the row a tooltip
  naming its branch and its lock or prune reason.
- Move the tooltip out of the badge refresh, which reassigned it on
  every window activation and so would have erased whatever the row
  had set for itself.
- Cover the whole chain with a test that runs `git worktree add` and
  `git worktree lock`, the only worktree test reading git's own output
  rather than a fixture that cannot notice git changing.

A worktree is a branch that happens to sit in its own directory, so it
belongs in one place in the sidebar; listing it as both a branch and a
directory stated the same fact twice under two different names.

- Name a worktree row after the branch it holds, keeping the directory
  in the tooltip with the lock and prune state
- Keep a branch held elsewhere out of BRANCHES, in both directions, as
  the worktree snapshot only lands after the branches are built
- Answer with that branch from the row, so the ref menu builds and
  already offers to open the worktree rather than to check it out
- Give every row its name on hover, so a truncated one stays readable,
  and reopen the worktree on a double click as the branch rows did

A branch listed under BRANCHES can no longer be held by another
worktree, so everything that answered for that case had become
unreachable.

- Take the worktree path and the icon override out of the cell builder,
  which now reads its icon off the item like every other row
- Drop the cell's worktreePath property and the tooltip it wrote
- Let a double click on a branch row check it out, which is the only
  thing it can still mean