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
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