Allow ReadTreeTest to test arbitrary Checkouts

Allow ReadTreeTest to test arbitrary Checkouts

ReadTreeTest was hardcoded to test WorkDirCheckout. Since we want
alternative checkout implementations (especially DirCacheCheckout)
this class has been refactored so that the tests can be reused
to test other implementations

The following changes have been done:
- abstract methods for checkout and prescanTwoTrees have been
  introduced. Parameters are only the two trees. As index we
  will implicitly use the current index of the repo.
- whenever tests needed a manipulated index before checkout
  and prescanTwoTrees it was ensured that the correct index was
  persisted (before we could use not-persisted instantiations of GitIndex
  passed as parameters to checkout, prescanTwoTrees
- abstract methods for getting updated, conflicting, removed entries
  resulting from the last checkout, prescanTwoTrees have been introduced
- an implementation for all these abstract methods using WorkDirCheckout
  has been added
- method to assert a certain state of the index and the working tree has
  been added

Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>
Change-Id: 	Icf177cf8043487169a32ddd72b6f8f9246a433f7