.arcconfig

templater: drop symbols which should be overridden by new 'ctx' (issue5612)

This problem is caused by impedance mismatch between the templater and the
formatter interface, which is that the template keywords are generally
evaluated dynamically, but the formatter puts static values into a template
mapping.

This patch avoids the problem by removing conflicting values from a mapping
dict when a 'ctx' is switched.

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