merge-ort: call diffcore_rename() directly

We want to pass additional information to diffcore_rename() (or some
variant thereof) without plumbing that extra information through
diff_tree_oid() and diffcore_std().  Further, since we will need to
gather additional special information related to diffs and are walking
the trees anyway in collect_merge_info(), it seems odd to have
diff_tree_oid()/diffcore_std() repeat those tree walks.  And there may
be times where we can avoid traversing into a subtree in
collect_merge_info() (based on additional information at our disposal),
that the basic diff logic would be unable to take advantage of.  For all
these reasons, just create the add and delete pairs ourself and then
call diffcore_rename() directly.

This change is primarily about enabling future optimizations; the
advantage of avoiding extra tree traversals is small compared to the
cost of rename detection, and the advantage of avoiding the extra tree
traversals is somewhat offset by the extra time spent in
collect_merge_info() collecting the additional data anyway.  However...

For the testcases mentioned in commit 557ac0350d ("merge-ort: begin
performance work; instrument with trace2_region_* calls", 2020-10-28),
this change improves the performance as follows:

                            Before                  After
    no-renames:       13.294 s ±  0.103 s    12.775 s ±  0.062 s
    mega-renames:    187.248 s ±  0.882 s   188.754 s ±  0.284 s
    just-one-mega:     5.557 s ±  0.017 s     5.599 s ±  0.019 s

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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