ui: factor out function that writes data to fout/ferr with labeling
I'm thinking of adding an option to send status messages to stderr (or a
dedicated command-server channel) so that structured output (e.g. JSON)
would never be interleaved with non-formatter output. A unified write()
interface helps to do that.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}