"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()
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