clone-bundles: add an option to generate bundles in the background
This is what the "on-change" version have been doing from the start, it seems
useful to also offer this option in the command.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}