cache: replace asizeof() call with simpler heuristic

The asizeof() introduced a very heavy overhead which made swhfs unusable
when relying on many asizeof calls (eg: storing history exploration in
the direntry cache with a history containing >1000 commits).

The heuristic to calculate the size of a FuseEntry is based on
experimental results from repositories with various history sizes (10K,
100k, ...)