Don't close pack fd when free'ing pack windows

Now that close_one_pack() has been introduced to handle file
descriptor pressure, it is not strictly necessary to close the
pack file descriptor in unuse_one_window() when we're under memory
pressure.

Jeff King provided a justification for leaving the pack file open:

   If you close packfile descriptors, you can run into racy situations
   where somebody else is repacking and deleting packs, and they go away
   while you are trying to access them. If you keep a descriptor open,
   you're fine; they last to the end of the process. If you don't, then
   they disappear from under you.

   For normal object access, this isn't that big a deal; we just rescan
   the packs and retry. But if you are packing yourself (e.g., because
   you are a pack-objects started by upload-pack for a clone or fetch),
   it's much harder to recover (and we print some warnings).

Let's do so (or uh, not do so).

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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