Fix completion issues

[mateusauler]

This PR fixes:

• Fish completions would only run once per shell session.
• Every shell completion would not accept worktrees with spaces in their basename.

This PR adds:

• New names sub-command that lists out the basename of every worktree.
• Dependency on xargs.

This PR also obsoletes #14.

[EliSauder]

Just a heads up, the added names command fails when on macos.

Error: xargs: Unknown option 'I{}' (see "xargs --help")

You should be able to change it to the below snippet without changing functionality on any platform.

worktree_list_names() {
  if git rev-parse --git-dir &> /dev/null; then
    git worktree list --porcelain \
      | awk '/^worktree / { sub("worktree ", ""); print; }' \
      | xargs basename
  fi
}

[mateusauler]

Thanks @EliSauder! I added your suggestion into a new release on my fork.