revlog: fix the `null_item` attribute for V0 This is not a proper index tuple. Differential Revision: https://phab.mercurial-scm.org/D10645

revlog: create a create `null_item` attribute for V0 This value is currently wrong, but at least it is now -obviously- wrong. Differential Revision: https://phab.mercurial-scm.org/D10644

revlog: improve documentation of the entry tuple The code in revlog, and outside revlog directly use the index's entry tuple, with direct integer indexing. This is a voluntary trade off to obtains better performance from the Python code at the expense of the developers sanity. Let's at least have a clear and central documentation about what this tuple is about. Differential Revision: https://phab.mercurial-scm.org/D10643

revlog: simplify entry update logic in `rewrite_sidedata` We don't need to rebuild the full entry. So lets just gather the data we need to pass to `replace_sidedata_info`. Differential Revision: https://phab.mercurial-scm.org/D10642

revlog: simplify the replace_sidedata_info code We already know how to serialize and de-serialize and enty. So lets just do that and modify the entry tuple directly. This avoid having to duplicated binary operation in complicated code. Differential Revision: https://phab.mercurial-scm.org/D10641

revlog: use `rev` instead of `i` in replace_sidedata_info This is clearer. Differential Revision: https://phab.mercurial-scm.org/D10640

revlogv2: also keep track for the size of the "data" file This is useful to make sure we always start writing at the right location, without effort. Differential Revision: https://phab.mercurial-scm.org/D10632

revlogv2: track pending write in the docket and expose it to hooks The docket is now able to write pending data. We could have used a distinct intermediate files, however keeping everything in the same file will make it simpler to keep track of the various involved files if necessary. However it might prove more complicated for streaming clone. This will be dealt with later. Note that we lifted the stderr redirection in the test since we no longer suffer from "unkown working directory parent" message. Differential Revision: https://phab.mercurial-scm.org/D10631