'full' claims it will warm all of the caches that are known about, but this was
not the case - it did not actually warm the branchmap caches for subsets that we
haven't requested, or for subsets that are still considered "valid". By
explicitly writing them to disk, we can force the subsets for ex: "served" to be
written ("immutable" and "base"), making it cheaper to calculate "served" the
next time it needs to be updated.

Differential Revision: https://phab.mercurial-scm.org/D6710