exchangev2: add progress bar around manifest scanning
This can take a long time on large repositories. Let's add a progress
bar so we don't have long periods where it isn't obvious what is
going on.
Differential Revision: https://phab.mercurial-scm.org/D4859
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}