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A random hodge-podge of incorrect or out-of-date comments that I found: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