perf: invalidate less contradicted_incompatibilities (pubgrub-rs#170)

* Invalidate less contradicted_incompatibilities

* update comments

src/internal/core.rs

@@ -227,6 +227,7 @@ impl<P: Package, VS: VersionSet, Priority: Ord + Clone> State<P, VS, Priority> {
     ) {
         self.partial_solution.backtrack(decision_level);
         // Remove contradicted incompatibilities that depend on decisions we just backtracked away.
+        // Remove contradicted incompatibilities that depend on decisions we just backtracked away.
         self.contradicted_incompatibilities
             .retain(|_, dl| *dl <= decision_level);
         if incompat_changed {