diffcore-rename: simplify limit check

diffcore-rename: simplify limit check

diffcore-rename had two different checks of the form

    if ((a < limit || b < limit) &&
        a * b <= limit * limit)

This can be simplified to

    if (st_mult(a, b) <= st_mult(limit, limit))

which makes it clearer how we are checking for overflow, and makes it
much easier to parse given the drop from 8 to 4 variable appearances.

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