Improve odebug/SystemCommand debugging output.

[reitermarkus]

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Make odebug always output to stderr so it doesn't break normal output (e.g. when piping --json output).

Also add a new print_stdout: :debug option to SystemCommand which outputs the full command output to stderr when HOMEBREW_DEBUG/--debug is set.

Needed for Homebrew/homebrew-services#595.

[MikeMcQuaid]

Make odebug always output to stderr so it doesn't break normal output (e.g. when piping --json output).

I think this should be opt-in probably so it's grepable more easily but am open to thoughts from @Homebrew/maintainers if anyone else disagrees.

Also add a new print_stdout: :debug option to SystemCommand which outputs the full command output to stderr when HOMEBREW_DEBUG/--debug is set.

This is a great idea!

[carlocab]

Sending debug output to stderr makes sense to me, including doing so unconditionally. Fine with making it opt-out if someone needs it, but it's not something I'd personally use.

[MikeMcQuaid]

Ok, cool, fine with me, thanks @reitermarkus! A legit test failure needs fixed and then good to 🚢.

[reitermarkus]

Yeah, the test is a bit flaky, I assume due to stdout being buffered while stderr isn't. Will take another look later.