copies: create helper for getting all copies for changeset There are a few places where we get all the copies for a changeset (at least the {file_copies} template and in two places in `hg log --copies` code). These places currently call scmutil.getrenamedfn() to get a caching "getrenamed" function. They all use it in a similar way. We will be able to reuse more code by having a function for getting all the copies for a changeset. This patch introduces such a function. It uses it in the {file_copies} template to show that it works. It relies on the existing scmutil.getrenamedfn() for caching in the filelog-centric case. Differential Revision: https://phab.mercurial-scm.org/D6545