commandserver: switch logging facility to ui.log() interface
The "pager subcommand" message is removed since ui isn't accessible there.
I think that's okay as cmdtable[cmd]() will call attachio() and some debug
message will be printed.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}