repair: move ui.history-editing-backup to [rewrite] section Since we have the "rewrite" section for general history-editing options, the backup option should be there.

revlog: use separate variables to track version flags It wasn't obvious to me that "versionflags" is used both to define the default version+features value for new revlogs and to track the value read from a revlog. We rename the former use and add explicit assignment of "versionflags" later to differentiate between the two. Differential Revision: https://phab.mercurial-scm.org/D5564

revlog: inline opener options logic into _loadindex() We always call _loadindex() during __init__. But we also call _loadindex() as part of censorrevision(). Before, when reloading the index during censorrevision(), we would lose the configured mmapindexthreshold setting from the opener. By inlining the logic in _loadindex(), we ensure that opener options are always respected when loading the index. Differential Revision: https://phab.mercurial-scm.org/D5563

revlog: store mmaplargeindex as an instance attribute This makes it more consistent with everything else. It also makes it possible to move the opener/features processing into _loadindex(). Differential Revision: https://phab.mercurial-scm.org/D5562

revlog: always enable generaldelta on version 2 revlogs This commit starts the process of diverging version 2 revlogs from version 1 revlogs. generaldelta is a useful feature and has been enabled by default for ages. I can't think of a good reason why the feature should be disabled. Yes, it is true changelogs today don't have generaldelta enabled. But that's because they don't have delta chains enabled, so generaldelta makes no sense there. This commit makes generaldelta always enabled on version 2 revlogs. As part of this, one-off code in changelog.py mucking with revlog.version had to be made conditional on the revlog version, as we don't want to change revlog feature flags on version 2 revlogs. The fact this code exists is horrible and stems from revlog options being shared by the opener. We probably want a better API here. But that can wait for another patch. Differential Revision: https://phab.mercurial-scm.org/D5561

revlog: rename v to versionflags Single letter variables are harder to read. Differential Revision: https://phab.mercurial-scm.org/D5560

revlog: always process opener options I'm not sure when ``opener.options`` would ever be explicitly set to None. It is definitely not possible to construct a repo this way because ``localrepo.resolvestorevfsoptions()`` always returns a dict and ``localrepo.makelocalrepository()`` always sets ``opener.options`` to this value. Because we always execute this code now, if options are empty we defaulted to creating version 0 revlogs. So we had to change the code slightly to fall back to the default revlog version and flags. As astute reader will note that it is not possible to create version 0 revlogs now. However, I don't think it was possible before, as this required ``opener.options`` being unset, which I don't think was possible. I suspect this means our test coverage for version 0 revlog repositories is possibly non-existent! Since I don't see a config option to disable revlog v1, I'm not even sure if we had a way to create new repos with version 0 revlogs! Who knows. Differential Revision: https://phab.mercurial-scm.org/D5559

internals: minor rewriting of revlogs documentation In preparation for formalizing revlog version 2. Differential Revision: https://phab.mercurial-scm.org/D5558