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.
Now the description includes the variable scope, `set [-e] -[Ugl]`
completions only provide variables matching that scope, and completions
that shouldn't be modified are hidden from the user. Completions that
are often modified but rarely unset (`fish_*` variables) are omitted
from `set -e` completions.
A new helper function `__fish_seen_argument` has been added that makes
it easy to only provied completions for a specific flag.
