handle concurrent pruning of packed objects

handle concurrent pruning of packed objects

This patch causes read_sha1_file and sha1_object_info to re-examine the
list of packs if an object cannot be found. It works by re-running
prepare_packed_git() after an object fails to be found.

It does not attempt to clean up the old pack list. Old packs which are in
use can continue to be used (until unused by lru selection).  New packs
are placed at the front of the list and will thus be examined before old
packs.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>