revlogv2: preserve the compression mode on disk The value is still the same and still not treated, but now it exists one disk. Differential Revision: https://phab.mercurial-scm.org/D10648

revlog: use `format_version` to dispatch different version in index_get This is semantically more correct. Differential Revision: https://phab.mercurial-scm.org/D10647

revlog: add a "data compression mode" entry in the index tuple That will make it possible to keep track of compression information in the revlog index, opening the way to more efficient revision restoration (in native code, but the python usage is already defeating performance work). We start with adding a new entry to the index tuple, using a value matching the current behavior. We will introduce storage and other value in later changesets. Differential Revision: https://phab.mercurial-scm.org/D10646

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