copies: avoid unwanted side effect from one branch to another

Without this copy, change in a one descendant branch (With "remove" change
only) could affect computation on another descendant branches.

This was not caugh by the test because the test graph are "too simple". I
started writing more test in that regards, but I a submitting this changes
earlier because I want to get more code landed to allow other optimisation work
to happens.

Differential Revision: https://phab.mercurial-scm.org/D9416

mercurial/copies.py

@@ -354,7 +354,9 b' def _combine_changeset_copies('
                         source = prev[1]
                     newcopies[dest] = (c, source)
                 assert newcopies is not copies
-            if changes is not None:
+            if changes is not None and changes.removed:
+                if newcopies is copies:
+                    newcopies = copies.copy()
                 for f in changes.removed:
                     if f in newcopies:
                         if newcopies is copies: