merge-ort: update comments to modern testfile location

merge-ort: update comments to modern testfile location

In commit 919df3195553 (Collect merge-related tests to t64xx,
2020-08-10), merge related tests were moved from t60xx to t64xx.  Some
comments in merge-ort relating to some tricky code referenced specific
testcases within certain testfiles for additional information, but
referred to their historical testfile names; update the testfile names
to mention their modern location.

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