deltas: add a `debug.revlog.debug-delta` config option enable output
This provide a way to enable the code introduced in the previous changeset. This
will provide a large amount of output when applying a bundle with details about
each delta "computation".
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}