checkcopies: add a sanity check against false-positive copies When grafting a copy backwards through a rename, a copy is wrongly detected, which causes the graft to be applied inappropriately, in a destructive way. Make sure that the old file name really exists in the common ancestor, and bail out if it doesn't. This fixes the aggravated case of bug 5343, although the basic issue (failure to duplicate the copy information) still occurs.