Fix various issues found in comments

Fix various issues found in comments

A random hodge-podge of incorrect or out-of-date comments that I found:

  * t6423 had a comment that has referred to the wrong test for years;
    fix it to refer to the right one.
  * diffcore-rename had a FIXME comment meant to remind myself to
    investigate if I could make another code change.  I later
    investigated and removed the FIXME, but while cherry-picking the
    patch to submit upstream I missed the later update.  Remove the
    comment now.
  * merge-ort had the early part of a comment for a function; I had
    meant to include the more involved description when I updated the
    function.  Update the comment now.

Signed-off-by: Elijah Newren <newren@gmail.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>