pull: flush stdin after the `pull from` message
That message can end up being flushed after some stderr message in some case,
leading to confusing output.
Differential Revision: https://phab.mercurial-scm.org/D9506
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}