configitems: enable changegroup3 by default (unless using infinitepush)
The LFS extension requires this, and if it isn't enabled on the client (or the
LFS extension isn't loaded), a web client gets a 500 instead of a sensible error
message. Now it gets a different (client) error, but maybe it can be handled
more gracefully.
c0f11347b107 indicates that treemanifest repos have this issue
too.
29cfc474c5fd mentions gating this behind `experimental` so that the format
could change, but that was 7 years ago and we now have an experimental
`changegroup4` as well.
We can keep this as a config for the next cycle in case someone runs into an
unexpected problem, and then jettison it if the infinitepush bundle name changes
are either acceptable as-is or can be created differently.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}