sparse: use None as the sparse matcher value when disabled This create a clear signal for when the feature is unused. We could also create an `alwaysmatcher`, but using None is more explicit, so I went for it.

sparse: start moving away from the global variable for detection of usage Code is now checking if the repository using the sparse feature and that's it. Some code in `debugsparse` still rely on "global" state, as it apply sparse logic before updating the requirement. Cleaning that up is more work that we signed up for, but we could narrow the hack to that specific command.

test: fix test-sparse-revlog output after debugdeltachain change We need to get this test covered by the CI, in the meantime, here is a quick fix.