copies: fix duplicatecopies() with overlay context The reasoning for this check is in 78d760aa3607 (duplicatecopies: do not mark items not in the dirstate as copies, 2013-03-28). The check was then moved to workingfilectx in 754b5117622f (context: add workingfilectx.markcopied, 2017-10-15) and no corresponding check was added later when overlayworkingfilectx was added. Rather than adding the check there, this patch adds a more generic check on the callers side and removes the check in workingfilectx.markcopied(). Differential Revision: https://phab.mercurial-scm.org/D6380