Fix 'string length: Unknown option': add `--` before $subcommand
Fix count $subcommand always = 1 with `sudo` and `doas`:
give argv as array to __fish_complete_subcommand
[ci skip]
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.
Use clang/clang++'s own autocompletion support to complete arguments. It
is rather convoluted as clang generates autocompletions for a portion of
the current token rather than the entire token, e.g. while `--st` will
autocomplete to `--std=` (which is fine by fish), `--std=g` will
autocomplete to `gnu...` without the leading `--std=` which breaks fish'
support for the completion.
Additionally, on systems where clang/clang++ is the system compiler
(such as FreeBSD), it is very often for users to invoke a newer version
of clang/clang++ installed as clang[++]-NN instead of clang. Using a
monkey-patched version of `complete -p` to support that without breaking
(future) completions for commands like `clang-format`.
Closes#4174.
