tree-cache: correct the entry_count calculation

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.

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I have no idea wtf I was thinking with the current recount code, it makes no sense that we'd even need to recount, let alone that way. This time I've replaced the values in the tests by counting by hand.