rust-cpython: using MissingAncestors from Python code As precedently done with LazyAncestors on cpython.rs, we test for the presence of the 'rustext' module. incrementalmissingrevs() has two callers within the Mercurial core: `setdiscovery.partialdiscovery` and the `only()` revset. This move shows a significant discovery performance improvement in cases where the baseline is slow: using perfdiscovery on the PyPy repos, prepared with `contrib/discovery-helper <repo> 50 100`, we get averaged medians of 403ms with the Rust version vs 742ms without (about 45% better). But there are still indications that performance can be worse in cases the baseline is fast, possibly due to the conversion from Python to Rust and back becoming the bottleneck. We could measure this on mozilla-central in cases were the delta is just a few changesets. This requires confirmation, but if that's the reason, then an upcoming `partialdiscovery` fully in Rust should solve the problem. Differential Revision: https://phab.mercurial-scm.org/D5551