git_prompt: Only show untracked files in informative mode if asked

[faho]

This makes it so

1. The informative status can work without showing untracked
   files (previously it was disabled if bash.showUntrackedFiles was
   false)
2. If untrackedfiles isn't explicitly enabled, we use -uno, so git
   doesn't have to scan all the files.

In a large repository (like the FreeBSD ports repo), this can improve
performance by a factor of 5 or up.

---

The impetus for this is that our informative+vcs prompt already tries to disable untracked file display via the (undocumented) __fish_git_prompt_hide_untrackedfiles variable, so in that case we're just wasting time.

We could also make it so we recognize that option and skip untracked display (or make it use $__fish_git_prompt_status_order without untrackedfiles), but:

1. We've decided that it's not useful there, so we apparently don't like it in the prompt meant to showcase the informative git prompt!
2. This can be very costly and so should probably be enabled explicitly.

Part of #8979 (well, except for the terlar bit)

(we probably also want to make it so you can set $__fish_git_prompt_showuntrackedfiles to 0 to explicitly disable it)

TODOs:

• Changes to fish usage are reflected in user documentation/manpages.
• Tests have been added for regressions fixed
• User-visible changes noted in CHANGELOG.rst

[faho]

the (undocumented) __fish_git_prompt_hide_untrackedfiles variable

Here's my plan for the future:

1. Document $__fish_git_prompt_status_order
2. Read the git prompt boolean variables as 0/1 toggles instead of checking if they are set or not, so you can disable them
3. Remove __fish_git_prompt_hide_X, as it's now superfluous (and was only ever used with informative status)
4. (possibly rename the variables to without the dunderscore - $fish_git_prompt_show_untrackedfiles etc)