upgrade: document DELTAREUSEFULLADD in revlog.clone
The documentation of the `debugupgraderepo` command was updated when
`re-delta-fulladd` was introduced. However, the docstring of the `revlog.clone`
method was not.
Differential Revision: https://phab.mercurial-scm.org/D6900
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}