copies: extract value comparison in the python copy tracing

This mirror what we did in the Rust code. This is useful to prepare rework for
this comparison logic that we will need to handle more advanced chaining cases
(when merges are chained).

Differential Revision: https://phab.mercurial-scm.org/D9590

mercurial/copies.py

@@ -446,6 +446,12 b' def _combine_changeset_copies('
     return final_copies
 
 
+# constant to decide which side to pick with _merge_copies_dict
+PICK_MINOR = 0
+PICK_MAJOR = 1
+PICK_EITHER = 2
+
+
 def _merge_copies_dict(minor, major, isancestor, changes):
     """merge two copies-mapping together, minor and major
 
@@ -464,34 +470,35 b' def _merge_copies_dict(minor, major, isa'
         if other is None:
             minor[dest] = value
         else:
+            pick = _compare_values(changes, isancestor, dest, other, value)
+            if pick == PICK_MAJOR:
+                minor[dest] = value
+    return minor
+
+
+def _compare_values(changes, isancestor, dest, other, value):
+    """compare two value within a _merge_copies_dict loop iteration"""
             new_tt = value[0]
             other_tt = other[0]
+
             if value[1] == other[1]:
-                continue
+        return PICK_EITHER
             # content from "major" wins, unless it is older
             # than the branch point or there is a merge
             if new_tt == other_tt:
-                minor[dest] = value
-            elif (
-                changes is not None
-                and value[1] is None
-                and dest in changes.salvaged
-            ):
-                pass
-            elif (
-                changes is not None
-                and other[1] is None
-                and dest in changes.salvaged
-            ):
-                minor[dest] = value
+        return PICK_MAJOR
+    elif changes is not None and value[1] is None and dest in changes.salvaged:
+        return PICK_MINOR
+    elif changes is not None and other[1] is None and dest in changes.salvaged:
+        return PICK_MAJOR
             elif changes is not None and dest in changes.merged:
-                minor[dest] = value
+        return PICK_MAJOR
             elif not isancestor(new_tt, other_tt):
                 if value[1] is not None:
-                    minor[dest] = value
+            return PICK_MAJOR
                 elif isancestor(other_tt, new_tt):
-                    minor[dest] = value
-    return minor
+            return PICK_MAJOR
+    return PICK_MINOR
 
 
 def _revinfo_getter_extra(repo):