Merge branch 'nd/worktree-lock'

Merge branch 'nd/worktree-lock'

"git worktree prune" protected worktrees that are marked as
"locked" by creating a file in a known location.  "git worktree"
command learned a dedicated command pair to create and remove such
a file, so that the users do not have to do this with editor.

* nd/worktree-lock:
  worktree.c: find_worktree() search by path suffix
  worktree: add "unlock" command
  worktree: add "lock" command
  worktree.c: add is_worktree_locked()
  worktree.c: add is_main_worktree()
  worktree.c: add find_worktree()