merge-ort: drop unused parameters from detect_and_process_renames()

merge-ort: drop unused parameters from detect_and_process_renames()

This function takes three trees representing the merge base and both
sides of the merge, but never looks at any of them. This is due to
f78cf97617 (merge-ort: call diffcore_rename() directly, 2021-02-14).
Prior to that commit, we passed pairs of trees to diff_tree_oid(). But
after that commit, we collect a custom diff_queue for each pair in the
merge_options struct, and just run diffcore_rename() on the result. So
the function does not need to know about the original trees at all
anymore.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>