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
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