watchman: refactor transport connecting to unconfuse pytype
Pytype sees `self.proc` as potentially `None` here, even though it's set by the
`_connect()` logic. Instead of asserting, simply use the process returned by
that method (which it sets into `self.proc` before returning, so there's no
functional change here).
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}