revlog: make the index always return the same tuple It is simpler to manage the diferrence in on disk format in the internal index code itself and lets the rest of the code always handle the same object. This will become even more important when the data we store will be entirely different (for example the changelog does not need the "linkrev" field. We start with item reading, we will deal with item writing in the next changesets. Differential Revision: https://phab.mercurial-scm.org/D10568