lookup: don't use "00changelog.i@None" when lookup of prefix fails
We were shadowing the "node" variable, so we always passed None to the
LookupError instead of the node we meant to pass.
(This showed up in py3 tests since py3 doesn't like to format None
using "%s".)
Differential Revision: https://phab.mercurial-scm.org/D6661
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}