copies-rust: refactor the "deletion" case

We rearrange the code to single out the case where information need to be
overwritten on both side of the merge. This open the way to better dealing with
this case.

Differential Revision: https://phab.mercurial-scm.org/D9651

rust/hg-core/src/copy_tracing.rs

@@ -510,22 +510,35 b' fn chain_changes('
                 // propagate this information when merging two
                 // InternalPathCopies object.
                 let deleted = path_map.tokenize(deleted_path);
-                match &mut p1_copies {
-                    None => (),
-                    Some(copies) => {
-                        copies.entry(deleted).and_modify(|old| {
-                            old.mark_delete(current_rev);
-                        });
-                    }
+
+                let p1_entry = match &mut p1_copies {
+                    None => None,
+                    Some(copies) => match copies.entry(deleted) {
+                        Entry::Occupied(e) => Some(e),
+                        Entry::Vacant(_) => None,
+                    },
+                };
+                let p2_entry = match &mut p2_copies {
+                    None => None,
+                    Some(copies) => match copies.entry(deleted) {
+                        Entry::Occupied(e) => Some(e),
+                        Entry::Vacant(_) => None,
+                    },
                 };
-                match &mut p2_copies {
-                    None => (),
-                    Some(copies) => {
-                        copies.entry(deleted).and_modify(|old| {
-                            old.mark_delete(current_rev);
-                        });
+
+                match (p1_entry, p2_entry) {
+                    (None, None) => (),
+                    (Some(mut e), None) => {
+                        e.get_mut().mark_delete(current_rev)
                     }
-                };
+                    (None, Some(mut e)) => {
+                        e.get_mut().mark_delete(current_rev)
+                    }
+                    (Some(mut e1), Some(mut e2)) => {
+                        e1.get_mut().mark_delete(current_rev);
+                        e2.get_mut().mark_delete(current_rev);
+                    }
+                }
             }
         }
     }