Merge branch 'es/worktree-duplicate-paths'
The same worktree directory must be registered only once, but
"git worktree move" allowed this invariant to be violated, which
has been corrected.
* es/worktree-duplicate-paths:
worktree: make "move" refuse to move atop missing registered worktree
worktree: generalize candidate worktree path validation
worktree: prune linked worktree referencing main worktree path
worktree: prune duplicate entries referencing same worktree path
worktree: make high-level pruning re-usable
worktree: give "should be pruned?" function more meaningful name
worktree: factor out repeated string literal