config: use 'experimental.evolution.exchange'
Extract 'experimental.evolution' = exchange as
'experimental.evolution.exchange'.
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.exchange'.
Differential Revision: https://phab.mercurial-scm.org/D1151