Factor2[poly] factors a polynomial in a standard
way.
Factor2 works sometimes better than Factor on
polynomials involving rationals with sums in the denominator.
Factor2 uses Factor internally and is in
general slower than Factor.
(a - x) (b - x)
{Factor2[%], Factor[%]}(a-x) (b-x)
\{(a-x) (b-x),-((a-x) (x-b))\}
ex = Expand[(a - b) (a + b)]a^2-b^2
Factor[ex](a-b) (a+b)
Factor2[ex]a^2-b^2