[vim] Do not escape % when using system() instead of !
[vim] Do not escape % when using system() instead of !

Close https://github.com/junegunn/fzf.vim/issues/309

janlazo/fzf,tomfitzhenry/pkg-fzf,wilywampa/fzf,janlazo/fzf,junegunn/fzf,stuha/wbr-zsh,infokiller/fzf,wilywampa/fzf,tomfitzhenry/pkg-fzf,infokiller/fzf,tomfitzhenry/pkg-fzf,infokiller/fzf,junegunn/fzf,junegunn/fzf,janlazo/fzf,wilywampa/fzf