movedirstate: get copies from dirstate before setting parents

Setting dirstate parents can modify the copies recorded in the
dirstate when there are two dirstate parents. I don't think we ever
call movedirstate() when there is more than one parent, but it seems
clearer to get the copies from the dirstate first anyway.

Differential Revision: https://phab.mercurial-scm.org/D7800

mercurial/scmutil.py

@@ -1431,8 +1431,8 b' def movedirstate(repo, newctx, match=Non'
     """
     oldctx = repo[b'.']
     ds = repo.dirstate
+    copies = dict(ds.copies())
     ds.setparents(newctx.node(), nullid)
-    copies = dict(ds.copies())
     s = newctx.status(oldctx, match=match)
     for f in s.modified:
         if ds[f] == b'r':