storage: introduce a `revlog.reuse-external-delta` config
This option goes a bit further and provides a way to get the same behavior as the
`re-delta-all` optimisation from `hg debugupgraderepo`.
The effect of the option is a bit hard to test as we do not have multiple diff
algorithm at hand. However, we at least make sure the code path run.
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}