flagutil: create a `mercurial.revlogutils.flagutil` module The flagprocessings logic is duplicated in 2 extra places, and usually in a less robust flavor. This is a maintenance nightmare that I would like to see cleaned up. To do so I am creating a `flagutil` module to move flag processings related code and make it easily reusable by other code.

rawdata: register the method for `ifiledata` The interface have a `revision(..., raw=False)` method so it should get a `rawdata` one. I am not sure why nothing complained about the lack of it earlier.

rawdata: implement the method for `unionrepo` too This is required for all implementations.

rawdata: implement the method for `remotefilelog` too This is needed for all storage implementations.