delta-find: split the "sparse" part of `_pre_filter_rev` in a method Since `_pre_filter_rev` contains logic from various sources of constraint, we start splitting is in subfunction to clarify and document the grouping.

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "phabricator.uri": "https://phab.mercurial-scm.org/",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}