copies-rust: extract the processing of a single copy information

This will make it easy to process copy from both p1 and p2 in the same
`add_from_changes` call.

Differential Revision: https://phab.mercurial-scm.org/D9648

rust/hg-core/src/copy_tracing.rs

@@ -503,6 +503,42 fn add_from_changes(
     for action in changes.iter_actions(parent) {
         match action {
             Action::Copied(path_dest, path_source) => {
+                add_one_copy(
+                    current_rev,
+                    &mut path_map,
+                    &mut copies,
+                    &base_copies,
+                    path_dest,
+                    path_source,
+                );
+            }
+            Action::Removed(deleted_path) => {
+                // We must drop copy information for removed file.
+                //
+                // We need to explicitly record them as dropped to
+                // propagate this information when merging two
+                // InternalPathCopies object.
+                let deleted = path_map.tokenize(deleted_path);
+                copies.entry(deleted).and_modify(|old| {
+                    old.mark_delete(current_rev);
+                });
+            }
+        }
+    }
+    copies
+}
+
+// insert one new copy information in an InternalPathCopies
+//
+// This deal with chaining and overwrite.
+fn add_one_copy(
+    current_rev: Revision,
+    path_map: &mut TwoWayPathMap,
+    copies: &mut InternalPathCopies,
+    base_copies: &InternalPathCopies,
+    path_dest: &HgPath,
+    path_source: &HgPath,
+) {
                 let dest = path_map.tokenize(path_dest);
                 let source = path_map.tokenize(path_source);
                 let entry;
@@ -529,21 +565,6 fn add_from_changes(
                     }
                 }
             }
-            Action::Removed(deleted_path) => {
-                // We must drop copy information for removed file.
-                //
-                // We need to explicitly record them as dropped to
-                // propagate this information when merging two
-                // InternalPathCopies object.
-                let deleted = path_map.tokenize(deleted_path);
-                copies.entry(deleted).and_modify(|old| {
-                    old.mark_delete(current_rev);
-                });
-            }
-        }
-    }
-    copies
-}
 
 /// merge two copies-mapping together, minor and major
 ///