Informative version of __fish_git_prompt_show_upstream

Configuration:

    set -g __fish_git_prompt_showupstream "informative"
    set -g __fish_git_prompt_char_upstream_ahead "↑" 
    set -g __fish_git_prompt_char_upstream_behind "↓" 
    set -g __fish_git_prompt_char_upstream_prefix ""