The entry_count field is the amount of index entries covered by a
particular cache entry, that is how many files are there (recursively)
under a particular directory.
The current code that attemps to do this is severely defincient and is
trying to count the amount of children, which always comes up to zero.
We don't even need to recount, since we have the information during the
cache creation. We can take that number and keep it, as we only ever
invalidate or replace.
Keeping the cache around after read-tree is only one part of the
optimisation opportunities. In order to share the cache between program
instances, we need to write the TREE extension to the index.
Do so, taking the opportunity to rename 'entries' to 'entry_count' to
match the name given in the format description. The included test is
rather trivial, but works as a sanity check.
