tests: update narrowspec when narrowspec, not dirstate, is accessed test-narrow-expanddirstate.t mimics a Google-internal extension that updates the narrowspec whenever the dirstate is accessed. Since 1d09ba0d2ed3 (narrow: move remaining narrow-limited dirstate walks to core, 2018-10-01) and a few commits before it, we no longer restrict repo.dirstate.walk() to the narrowspec. It is instead done at a higher level (e.g. context.status()). We were running into problems with the Google-internal extension when importing those commits. The issue was that the narrowspec was read before the first dirstate access. I believe the right fix is to instead update the narrowspec when trying to read it (not when reading the dirstate), so that's what this patch does. Differential Revision: https://phab.mercurial-scm.org/D5275