This runs build_tools/style.fish, which runs clang-format on C++, fish_indent on fish and (new) black on python.
If anything is wrong with the formatting, we should fix the tools, but automated formatting is worth it.
This created another local version of the variable just for the if-block.
Can't say I love the space prefix, but then I think we have too many
of these modes anyway.
If you use these to figure out if there _are_ staged files, or dirty
or whatever, you currently need to check the output, which relies on
the configured character.
Instead, we let them also return a useful status.
Notably, this is *not* simply the status of the git call.
__fish_git_prompt_X returns 0 if the repo is X.
This works for untracked, but the "diff" things return 1 if there is a
diff, so we invert the status for them.
See #5748.
[ci skip]
This exposes it more, since it's quite an important function.
We should do the same with the other vcs functions.
We leave a compatibility shim in place for now.
