debugdiscovery: only list common heads on verbose
The list of common heads is only part of the useful information. In addition on
repository with many heads, the information is very not helpful (just fill a
couple of screen with hash). As a result we hide it behind a --verbose flag.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}