I hope this doesn't impact speed much -- it does have to pull out a value
from Annex state every time it accesses the branch now.
The test case I dropped has never caught any problems that I can remember,
and would have been rather difficult to convert.
Slightly tricky as they are not normal UUIDBased logs, but are instead maps
from (uuid, chunksize) to chunkcount.
This commit was sponsored by Frank Thomas.
