stack: use repo.revs() instead of revsetlang.formatspec() + scmutil.revrange()
Using scmutil.revrange() it's possible to use multiple revsets at the same
time, but we're not using that functionality in stack.
I thought maybe that function could be used to make stack definition
customizable (by combining various parts into one set), but scmutil.revrange()
gives the union of all provided revsets, which is not very useful in stack's
case (we want "and" between parts, not "or").