fish_git_prompt: Fix changing show_informative_status

If you changed $__fish_git_prompt_show_informative_status, it
triggered a variable handler, which erased the chars, but neglected to
unset $___fish_git_prompt_init, so we just kept chugging along with
empty characters.

What's the hardest thing in CS again? Cache something something?

[ci skip]

share/functions/fish_git_prompt.fish

@@ -802,6 +802,8 @@ function __fish_git_prompt_repaint $varargs --description "Event handler, repain
             for name in cleanstate dirtystate invalidstate stagedstate stashstate stateseparator untrackedfiles upstream_ahead upstream_behind
                 set -e ___fish_git_prompt_char_$name
             end
+            # Clear init so we reset the chars next time.
+            set -e ___fish_git_prompt_init
         end
 
         commandline -f repaint 2>/dev/null