Enable comparaison of SHA1Ref object with an other DataRef
Support for the Null Type as a possible Git File.

The octal number is used inside logs and is useful for identification of
file addition or deletion. We use it during the parse operation of the log
entry for validation and future export.
We don't need to find the last modified time for this strategy.
Add more comments for the RepositoryHelper
Improve the file registration for new files.