rust-status: only involve ignore mechanism when needed
This prevents unnecessary fallbacks to Python, improving performance for
`hg update` for instance.
On Mozilla-Central a noop update goes from 1.6s down to 700ms.
Differential Revision: https://phab.mercurial-scm.org/D8315
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}