rhg: Add an allow-list of ignored extensions
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.
Differential Revision: https://phab.mercurial-scm.org/D10188
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