changegroup: add source parameter to generatemanifests
Extensions, like remotefilelog, will want to look at the source of a pull when
determining what manifests to add to a changegroup. For instance, on push they
will include everything, while on pull they won't.
Differential Revision: https://phab.mercurial-scm.org/D686
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}