Because rhg doesn’t know how a Python extension would affect
behavior it implements in Rust, when an unsupported extension
is enabled it conservatively falls back to Python-based hg.
However many users will have unsupported extensions enabled in practice.
Maybe they don’t actually affect rhg behavior, but we don’t know.
This adds a rhg.ignored-extensions configuration that lets
users list extensions that rhg can safely ignore and proceed even
if they’re not supported in Rust.
Maybe put the hint in (…) as we do for other hint in Mercurial.
(and maybe most this to an explicite hint attribute.