fix git in a directory containing spaces

Quick fix for #80.
Another one would be:

    -    if [[ -n "$topdir" ]] && [[ "$(basename $topdir)" == '.git' ]]  && [[ ! -z "$(git branch)" ]] ; then
    +    if [[ -n "$topdir" ]] && [[ "$(basename "$topdir")" == '.git' ]]  && [[ ! -z "$(git branch)" ]] ; then

but i'm not sure it will work with zsh.