.arcconfig

discovery: prevent deleting items from a dictionary

Removing elements from Python dictionary is expensive. So let's prevent adding
them instead.

I added a newline to make code look a bit better.

Differential Revision: https://phab.mercurial-scm.org/D6146

{
    "conduit_uri": "https://phab.mercurial-scm.org/api",
    "repository.callsign": "HG",
    "arc.land.onto.default": "@",
    "base": "hg:.^"
}