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.
Fixes some potentially unsafe uses of direct substitution into regex
expressions and also switches some completions to regex-based now that
there is a safe way of using it.
Executes `whatis` safely, returns at most one line, and strips the name
of the command from the start of line, returning a value fit for use as
the description parameter for a completion argument value.