repoview: add a test that access actual changeset data of `null`

Ideally, we would not trigger filtering here. However some work needs to happens
first.

Differential Revision: https://phab.mercurial-scm.org/D7482

tests/test-repo-filters-tiptoe.t

@@ -50,3 +50,10 b' Getting the node of `null`'
   $ cd test-repo
   $ hg log -r null -T "{node}\n"
   0000000000000000000000000000000000000000
+
+Getting basic changeset inforation about `null`
+
+  $ hg log -r null -T "{node}\n{date}\n"
+  debug.filters: computing revision filter for "visible"
+  0000000000000000000000000000000000000000
+  0.00