rebase: use hard-coded template for one-line commit description This is to prepare for making making the one-line summary customizable. The template ended up pretty complicated because of the conditional output of "(<bookmarks etc>)". Maybe we can simplify the template later. Differential Revision: https://phab.mercurial-scm.org/D9250

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