Implement a Dircache checkout (needed for merge)

Implement a Dircache checkout (needed for merge)

Implementation of a checkout (or 'git read-tree') operation which
works together with DirCache. This implementation does similar things
as WorkDirCheckout which main problem is that it works with deprecated
GitIndex. Since GitIndex doesn't support multiple stages of a file
which is required in merge situations this new implementation is
required to enable merge support.

Change-Id: I13f0f23ad60d98e5168118a7e7e7308e066ecf9c
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>