rust-clippy: use `write_all` since we're not expecting a partial write
Clippy caught this. This is a good lint in general, but here this was not
really a bug in this example code.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}