Merge branch 'sg/completion-worktree'

The command line completion (in contrib/) learned to complete subcommands and arguments to "git worktree". * sg/completion-worktree: completion: list paths and refs for 'git worktree add' completion: list existing working trees for 'git worktree' subcommands completion: simplify completing 'git worktree' subcommands and options completion: return the index of found word from __git_find_on_cmdline() completion: clean up the __git_find_on_cmdline() helper function t9902-completion: add tests for the __git_find_on_cmdline() helper