copies: extract function for deciding whether to use changeset-centric algos

Authored by martinvonz.

Description

copies: extract function for deciding whether to use changeset-centric algos

We'll eventually have a "experimental.copies.read-from=changeset-only"
option too and I don't want to spread the logic for determining if we
should use changeset-centric of filelog-centric algorithms.

Differential Revision: https://phab.mercurial-scm.org/D6163