worktree: teach `list` to annotate locked worktree

worktree: teach `list` to annotate locked worktree

The "git worktree list" shows the absolute path to the working tree,
the commit that is checked out and the name of the branch. It is not
immediately obvious which of the worktrees, if any, are locked.

"git worktree remove" refuses to remove a locked worktree with
an error message. If "git worktree list" told which worktrees
are locked in its output, the user would not even attempt to
remove such a worktree, or would realize that
"git worktree remove -f -f <path>" is required.

Teach "git worktree list" to append "locked" to its output.
The output from the command becomes like so:

    $ git worktree list
    /path/to/main             abc123 [master]
    /path/to/worktree         456def (detached HEAD)
    /path/to/locked-worktree  123abc (detached HEAD) locked

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Rafael Silva <rafaeloliveira.cs@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>