copies-rust: record overwrite when merging

When detecting fresh value (from current rev) overwriting older ones during
merges, we also record this overwrite for to help potential future comparison.

This does really have any performance effect right now, but it get use closer to
be able to drop all `is_ancestors` when merging.

Differential Revision: https://phab.mercurial-scm.org/D9499

rust/hg-core/src/copy_tracing.rs

@@ -731,11 +731,13 fn compare_value<A: Fn(Revision, Revisio
         } else {
             // The last value comes the current merge, this value -will- win
             // eventually.
+            oracle.record_overwrite(src_minor.rev, src_major.rev);
             MergePick::Major
         }
     } else if src_minor.rev == current_merge {
         // The last value comes the current merge, this value -will- win
         // eventually.
+        oracle.record_overwrite(src_major.rev, src_minor.rev);
         MergePick::Minor
     } else if src_major.path == src_minor.path {
         // we have the same value, but from other source;