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.
Instead, attempt to extract the message that _would_ be displayed on
execution of `./configure --help` by relying on some markers present in
autoconf-generated configure files.
As measured with 'hyperfine' on a laptop running in reduced frequency
power savings mode, `fish -c "__fish_parse_configure ./configure"`
runtime dropped from ~1.25s to ~0.8ms, which is inline with the
previously observed ~350ms execution time for `./configure --help`.
fish's own startup time is approximately 75ms before parsing begins.
Still very slow, but much better.
This relies on the new `read --line/-L` support as an entire parser for
the output of `./configure --help` was written in fishscript. Also
doesn't work without 72f32e6d8a7905b064680ec4b578c41dea62bf84.
The completion script is slow... a function of both the autotools
configure script itself being written in a shell script combined with a
fishscript output parser.
fish's own `./configure --help` takes around 350ms to execute, while
`__fish_parse_configure ./configure` (which runs that behind the scenes)
takes around 660ms to run, all-in-all - a not insignificant overhead.
Output can be cached (based off of ./configure hash or mtime) in the
future if this is a big deal.
