discovery: add a discovery.grow-sample.rate
This allow to control the effect of the growth rate on the discovery process
while doing analysis.
Differential Revision: https://phab.mercurial-scm.org/D9799
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"phabricator.uri": "https://phab.mercurial-scm.org/",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}