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