Added caching for loose object lookup during pack indexing

On Windows systems, file system lookup is a slow operation, so
checking each object if it exists during indexing (after receiving
the pack) could take a siginificant time. This patch introduces
CachedObjectDirectory that pre-caches lookup results.

Bug: 300397
Change-Id: I471b93f9bb3ee173eb37cae1d75e9e4eb49985e7
Signed-off-by: Constantine Plotnikov <constantine.plotnikov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>