store: speed up read and write of large fncache files
In my tests of an fncache containing 300,000 entries, this improves
read time from 567ms to 307, and write time from 1328ms to 533.
These numbers aren't so great, since the fncache file is only 17MB
in size, but they're an improvement.
.. Common link and substitution definitions.
.. |hg(1)| replace:: **hg**\ (1)
.. _hg(1): hg.1.html
.. |hgrc(5)| replace:: **hgrc**\ (5)
.. _hgrc(5): hgrc.5.html
.. |hgignore(5)| replace:: **hgignore**\ (5)
.. _hgignore(5): hgignore.5.html