RefDirectory was not using FileSnapshot correctly in all places. This
is fixed with this commit. Additionally the constructors for the
different types of refs have been changed to take a FileSnapshot
instead of a modification time.
Change-Id: Ifb6a59e87e8b058a398c38cdfb9d648f0bad4bf8
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>