Make InMemoryRepository pack names globally unique

Make InMemoryRepository pack names globally unique

It was easy to create multiple packs with exactly the same name and
same DfsRepositoryDescription in a test, which can poison the
DfsBlockCache. The javadoc for DfsObjDatabase.newPack() explicitly
says pack names should be unique within an entire DFS, so do this by
making the packId AtomicInteger static.

Arguably, test writers shouldn't be doing things like putting
'new DfsRepositoryDescription("test")' in a setUp() method, but that's
a natural thing to do, and we don't document this restriction
anywhere.

Change-Id: I9477413ab3950d83b7d17e173fbc0a3e064896e3