hooks: only disable/re-enable demandimport when it's already enabled

This fixes an issue introduced in d7c28954d901 where, when disabling
demandimport while running hooks, it's inadvertently re-enabled even when
it was never enabled in the first place.

This doesn't affect normal command line usage of Mercurial; it only matters
when Mercurial is run with demandimport intentionally disabled.

--HG--
branch : stable