Fix a bug in which the same path could be registered under multiple
worktree entries if the path was missing (for instance, was removed
manually). Also, as a convenience, expand the number of cases in
which --force is applicable.
* es/worktree-forced-ops-fix:
doc-diff: force worktree add
worktree: delete .git/worktrees if empty after 'remove'
worktree: teach 'remove' to override lock when --force given twice
worktree: teach 'move' to override lock when --force given twice
worktree: teach 'add' to respect --force for registered but missing path
worktree: disallow adding same path multiple times
worktree: prepare for more checks of whether path can become worktree
worktree: generalize delete_git_dir() to reduce code duplication
worktree: move delete_git_dir() earlier in file for upcoming new callers
worktree: don't die() in library function find_worktree()