log-tree: drop unused commit param in remerge_diff()

log-tree: drop unused commit param in remerge_diff()

This function has never used its "commit" parameter since it was added
in db757e8b8d (show, log: provide a --remerge-diff capability,
2022-02-02).

This makes sense; we already have separate parameters for the parents
(which lets us redo the merge) and the oid of the result tree (which we
can then diff against the remerge result).

Let's drop the unused parameter in the name of clarity.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>