Vim.Buffer: Use |gv| when visualmode() is <C-v> (#192)

* Don't need to save/restore register value
  when visualmode() !=# "\<C-v>"
* Must use |gv| only when using |CTRL-V|
  because |gv| is the only way to get selected text
  when using <C-v>$ .

Thanks @syngan for pointing out.
https://github.com/vim-jp/vital.vim/issues/192#issuecomment-53209197