vim: simplify the GitFiles? preview command
vim: simplify the GitFiles? preview command

We don't need to wrap this line in its own `sh` invocation in any of the
shell environments that I use.

Also, explicitly diff against `origin/HEAD`. I don't know if this is any
different than just `origin`, but we may as well request the exact thing
we want.

jparise/dotfiles,jparise/dotfiles,jparise/dotfiles,jparise/dotfiles