requirements: no longer drop `generaldelta` requirement with revlogv2 A repository could use a mix of revlogv1 and revlogv2, making the requirements still necessary. Overall we should move away from the "requirements" file being used a way to configure the repository and stick to it "what do you need to access this repository". However this is a wider work for another time. In addition the logic we just dropped was confusing the `hg debugformat` command, breaking the upgrade code and inconsistent (eg: `sparse-revlog` is also implied by `revlogv2`). Finally, multiple other config option would imply the use of the `revlogv2` requirements, without drop the `generaldelta` one, leading to more inconsistency. Differential Revision: https://phab.mercurial-scm.org/D10612

config: drop the `format.exp-revlogv2.2` option It is oddly named and not used at all. The one used by the code is still `experimental.revlogv2`. So we drop that one option for consistency. We move associate documentation to the actual option. Differential Revision: https://phab.mercurial-scm.org/D10611

revlog: only use the `_indexfp` method for read operation This will avoid "other" code to not overlook `_writing` usage. We introduces private method dedicated to writing to make use the right option are always used. Differential Revision: https://phab.mercurial-scm.org/D10610

revlog: use `_writing` in `rewrite_sidedata` Special cases aren't special enough to break the rules. Differential Revision: https://phab.mercurial-scm.org/D10609

revlog: open files in 'r+' instead of 'a+' The code doing actual writing is already doing the necessary seeking, so we could safely use 'r+'. This make the file objecs usable in more situation, like updating the sidedata information during pull. revlog: forcibly move the file cursor at the right location before writing This is a paranoid change in case the changelog computation moved the cursors under our feets. This is not known to happens right now. Differential Revision: https://phab.mercurial-scm.org/D10608

revlog: pass a transaction object to `rewrite_sidedata` The `_writing` context need one, so we update the function signature before considering using `_writing` in rewrite_sidedata. Differential Revision: https://phab.mercurial-scm.org/D10607

revlog: rename variable in `rewrite_sidedata` to match other code Let's call the index file object and `ifh` and the data file object `dfh` as the rest of the revlog code. This will make future change clearer. Differential Revision: https://phab.mercurial-scm.org/D10606