revset: drop argument when it's None

getstack's definition is `getstack(repo, rev=None)`, so providing None
explicitly is unnecessary. Moreover, when x is not None, it's definitely not a
revision but a part of a parsed tree of revset arguments.

Differential Revision: https://phab.mercurial-scm.org/D6707

mercurial/revset.py

@@ -1695,7 +1695,7 b' def stack(repo, subset, x):'
     parent. (EXPERIMENTAL)
     """
     if x is None:
-        stacks = stackmod.getstack(repo, x)
+        stacks = stackmod.getstack(repo)
     else:
         stacks = smartset.baseset([])
         for revision in getset(repo, fullreposet(repo), x):