t6429: update comment to mention correct tool

t6429: update comment to mention correct tool

A comment at the top of t6429 mentions why the test doesn't exercise git
rebase or git cherry-pick.  However, it claims that it uses `test-tool
fast-rebase`.  That was true when the comment was written, but commit
f920b0289ba3 (replay: introduce new builtin, 2023-11-24) changed it to
use git replay without updating this comment.

We could potentially just strike this second comment, since git replay
is a bona fide built-in, but perhaps the explanation about why it focuses
on git replay is still useful.  Update the comment to make it accurate
again.

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