Previously, the "Help.Vim Tutor" menu was using `:!vimtutor -g &`, but a
change in Vim (v8.2.3502) broke that behavior, as the whole script will
get terminated as soon as `:!` finishes running. Fix that to use `:call
system('vimtutor -g &')` instead, since that still works. See
https://github.com/vim/vim/issues/8951 for more discussions.
ea193e8b5d
