DfsFsck: reduce memory usage during verifyIndex

DfsFsck: reduce memory usage during verifyIndex

Don't convert a lot of ObjectId to String stored in generic
java.util.HashSet.  This is a very expensive way to store objects.

Instead rely on "this" from the FsckPackParser to lookup information
about the objects in this pack file, which lets the verify code avoid
sorting the object list.

Use ObjectIdOwnerMap, which is the most efficient format JGit has
for storing lots of objects.

Change-Id: Ib68f93acb4d91b96d0a44c0612f704500d332ac1