Re: [PATCH v3 8/9] hook: introduce extensions.hookStdoutToStderr

> All hooks already redirect stdout to stderr with the exception of
> pre-push which has a known user who depends on the separate stdout
> versus stderr outputs (the git-lfs project).
>
> The pre-push behavior was a surprise which we found out about after
> causing a regression for git-lfs. Notably, it might not be the only
> exception (it's the one we know about). There might be more.
>
> This presents a challenge because stdout_to_stderr is required for
> hook parallelization, so run-command can buffer and de-interleave
> the hook outputs using ungroup=0, when hook.jobs > 1.
>
> Introduce an extension to enforce consistency: all hooks merge stdout
> into stderr and can be safely parallelized. This provides a clean
> separation and avoids breaking existing stdout vs stderr behavior.
>
> When this extension is disabled, the `hook.jobs` config has no
> effect for pre-push, to prevent garbled (interleaved) parallel
> output, so it runs sequentially like before.
>
> Alternatives I've considered to this extension include:
> 1. Allowing pre-push to run in parallel with interleaved output.
> 2. Always running pre-push sequentially (no parallel jobs for it).
> 3. Making users (only git-lfs? maybe more?) fix their hooks to read
>    stderr not stdout.
>
> Out of all these alternatives, I think this extension is the most
> reasonable compromise, to not break existing users, allow pre-push
> parallel jobs for those who need it (with correct outputs) and also
> future-proofing in case there are any more exceptions to be added.