copies: fast path no-op merge

If the two sides of the merge are the same (they come an unaltered common
ancestors) we don't need any merging.

Differential Revision: https://phab.mercurial-scm.org/D9415

mercurial/copies.py

@@ -366,6 +366,9 b' def _combine_changeset_copies('
             othercopies = all_copies.get(c)
             if othercopies is None:
                 all_copies[c] = newcopies
+            elif newcopies is othercopies:
+                # nothing to merge:
+                pass
             else:
                 # we are the second parent to work on c, we need to merge our
                 # work with the other.