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.
