config: use 'experimental.evolution.allowunstable' Extract 'experimental.evolution' = allowunstable as 'experimental.evolution.allowunstable'. We keep the new option in the 'experimental.evolution' namespace in order to stay coherent with other options ('experimental.evolution.bundle-obsmarker' and 'experimental.evolution.track-operation') ease the renaming as possibly 'evolution.allowunstable'. Differential Revision: https://phab.mercurial-scm.org/D1150