t6423: add an explanation about why one of the tests does not pass

t6423: add an explanation about why one of the tests does not pass

I had long since forgotten the idea behind this test and why it failed,
and took a little while to figure it out.  To prevent others from having
to spend a similar time on it, add an explanation in the comments.
However, the reasoning in the explanation makes me question why I
considered it a failure at all.  I'm not sure if I had a better reason
when I originally wrote it, but for now just add commentary about the
possible expectations and why it behaves the way it does right now.

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