git_prompt: Informative upstream with informative status

This makes $__fish_git_prompt_showupstream = "auto" mean "informative"
instead of "git" if $__fish_git_prompt_show_informative_status is set.
git_prompt: Show upstream with informative status

This makes $__fish_git_prompt_show_informative_status imply
$__fish_git_prompt_showupstream = "informative", while adding a none
option for showupstream to disable it if desired.
git_prompt: Change informative character defaults

This changes the defaults for several characters when
$__fish_git_prompt_show_informative_status is set so that the prompt
looks more like the typical informative prompt.

Before:  (master >1<2|+3#4*5%6)
After:   (master↑1↓2|●3✖4✚5…6)

The defaults were taken from magicmonty/bash-git-prompt
git_prompt: Make informative separator configurable

It reuses $__fish_git_prompt_char_stateseparator, since it has a
similar meaning and goes otherwise unused when
$__fish_git_prompt_show_informative_status is set.