delta: reuse _findsnapshot call from previous stage
Two different stage of the sparse-revlog logic needs the _findsnapshot data.
To avoid recomputing it twice, make it possible to reuse the first computation
in the second step.
example affected manifest write
before: 0.067141s
after: 0.064252s (-5%)
(total gain since start of series: 95%)
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}