revlog: directly use the Struct object for related operation The Struct object has all the piece we needs, so no need to duplicate information on the revlog itself. Differential Revision: https://phab.mercurial-scm.org/D10307

revlog: move the "index header" struct inside revlog.utils.constants The struct was previous called "version", but this is actually "version" + "flags". So header seems like a better name. The move to the `constants` module has the same motivation as the INDEX_ENTRY_V# ones. Differential Revision: https://phab.mercurial-scm.org/D10306

revlog: move the details of revlog "v2" index inside revlog.utils.constants the revlog module is quite large and this kind of format information would handy for other module. So let us start to gather this information about the format in a more appropriate place. We update various reference to this information to use the new "source of truth" in the process. Differential Revision: https://phab.mercurial-scm.org/D10305

revlog: move the details of revlog "v1" index inside revlog.utils.constants The revlog module is quite large and this kind of format information would handy for other module. So let us start to gather this information about the format in a more appropriate place. We update various reference to this information to use the new "source of truth" in the process. Differential Revision: https://phab.mercurial-scm.org/D10304

revlog: move the details of revlog "v0" index inside revlog.utils.constants the revlog module is quite large and this kind of format information would handy for other module. So let us start to gather this information about the format in a more appropriate place. Differential Revision: https://phab.mercurial-scm.org/D10303

revlog: add some comment in the header sections We are about to add more content so let us organise the existing content first. Differential Revision: https://phab.mercurial-scm.org/D10302