typing: add type annotations to the `mercurial.util.filestat` class
It's referenced in the `vfs` classes, so get this out of the way to help there.
The `TypeVar` definition and its usage was copied from the existing `util.pyi`
file.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}