stack: change foo#stack[:] revset to also include stack base

This makes the revset to be less "special" and it'll work like a regular slice of a list that has all stack revs in it starting with stack base (e.g. [s0, s1, s2, ...]).

But foo#stack revset stays the same, meaning you don't get stack base when you use it. So if you just want to have all stack revs sans s0, you should use e.g. foo#s instead of foo#s[:].

Note that foo#stack revset is not available on 5.3 and older versions of Mercurial. If you cannot upgrade, you need to exclude stack base explicitly with foo#s[1:].