copies: avoid reusing the same variable for two different copy dicts

"childcopies" is initally the copies the current changeset to one of
its children and then we reassign it with the copies from the start of
the chain to the child. Let's use different names for these two
things.

Differential Revision: https://phab.mercurial-scm.org/D6564

mercurial/copies.py

@@ -312,15 +312,15 b' def _changesetforwardcopies(a, b, match)'
                                if match(dst)}
             # Copy the dict only if later iterations will also need it
             if i != len(children[r]) - 1:
-                copies = copies.copy()
-            if childcopies:
-                childcopies = _chain(copies, childcopies)
+                newcopies = copies.copy()
             else:
-                childcopies = copies
+                newcopies = copies
+            if childcopies:
+                newcopies = _chain(newcopies, childcopies)
             for f in childctx.filesremoved():
-                if f in childcopies:
-                    del childcopies[f]
-            heapq.heappush(work, (c, parent, childcopies))
+                if f in newcopies:
+                    del newcopies[f]
+            heapq.heappush(work, (c, parent, newcopies))
     assert False
 
 def _forwardcopies(a, b, match=None):