.arcconfig

export: add -B option to select a bookmark

Just like `hg email -B`, `hg strip -B`, supports -B in export to select a list
of changesets reachable from a bookmark.

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