dgit: unpack_playtree_need_cd_work: Make idempotent

dgit: unpack_playtree_need_cd_work: Make idempotent

This won't be able to recover from a truly-messed-up "work" but this
is good enough not to have to explicitly arrange for each place that
uses it to know whether it has been done already.

Checking in the filesystem is OK because mktree_in_ud_here calls
playtree_setup which expects the playground to have been set up
already - ie, all our callers have called prep_ud already.
Add a comment about this requirement.

No overall functional change.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>