Constant WORKTREE_LOCK 

pub const WORKTREE_LOCK: &str = "worktree.lock";

Basename of the repo-level lock that serialises worktree/index read-modify-write commands (add, rm, commit, merge, checkout, rebase, cherry-pick, stash, sparse-checkout).

Ref-only mutations (branch/tag) and config-only mutations do not take this lock — they rely on ref-CAS / atomic-config writes instead.