When using a DfsInserter for high-throughput insertion of many
objects (analogous to git-fast-import), we don't necessarily want to
do a random object lookup for each. It'll be faster from the
inserter's perspective to insert the duplicate objects and let a later
GC handle the deduplication.
Change-Id: Ic97f5f01657b4525f157e6df66023f1f07fc1851