InMemoryRepository: Ensure new ref targets exist in the repo

InMemoryRepository: Ensure new ref targets exist in the repo

ObjectInserter recently learned to read back inserted objects before
they have been flushed. It is in general unsafe to create refs to such
objects, but it is now much more possible to do so, by passing "new
RevWalk(inserter.newReader())" into RefUpdate#execute(RevWalk).

We can't change the RefUpdate interface to remove execute(RevWalk);
nor would we necessarily want to, for performance reasons. And in any
case, RefUpdate#safeParse explicitly ignores MissingObjectExceptions.
But we can enforce object existence in InMemoryRepository, which will
allow callers using this class in their tests to ensure they are using
the RefDatabase correctly.

Change-Id: I5c696ba23bcd2a536a0512fa7f5b6130961905c5