copies: split up _chain() in naive chaining and filtering steps The function now has two clearly defined steps. The first step is the actual chaining. This step is very cheap. The second step is filtering out invalid copies. This step is expensive. For changeset-centric copy tracing, I want to do the filtering step only at the end. This patch prepares for that. Differential Revision: https://phab.mercurial-scm.org/D6418