revlog: inline opener options logic into _loadindex()
We always call _loadindex() during init. But we also call
_loadindex() as part of censorrevision(). Before, when reloading
the index during censorrevision(), we would lose the configured
mmapindexthreshold setting from the opener. By inlining the
logic in _loadindex(), we ensure that opener options are always
respected when loading the index.
Differential Revision: https://phab.mercurial-scm.org/D5563