showconfig: use set to filter sections and entry names
Before, an entry matching the specified section could be printed twice if the
selector wasn't unique.
"sections" and "items" are renamed because it's hard to distinguish "sections"
from the loop variable "section".
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}