revsets: add commonancestors revset
This is a method to reproduce "::x and ::y" such that a set can be sent
in. For instance, it'd be convenient to have "::heads()" work like this
but that already means "::x + ::y + ..." for each element in the
Therefore, we add the "commonancestors" method to mean "::x and ::y ..."
for each head in the given set.