typing: add type hints to the prompt methods in mercurial/ui.py
The @overloads allow for the callers that pass a non-None `default` to not have
to worry about handling a None return to appease pytype.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}