rhg: support "!" syntax for disabling extensions
This makes it so that calls in test-log.t do not fall back immediately because
of the disabled extension, instead going through the CLI parsing code, which
breaks because of invalid UTF-8 in a flag.
I *think* clap 3.x+ supports this? I'm not sure, and we have to upgrade the
minimum Rust version to use clap 3.x anyway which is out of scope for this
series, so let's just kick that can down the road a little bit.
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