context: extract partial nodeid lookup method to scmutil
We will add another caller soon, and there's a non-obvious reason to
use the unfiltered repo that we don't want to copy across the code
base.
Differential Revision: https://phab.mercurial-scm.org/D3189
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}