sidedata: add a 'side-data' repository feature and use it Most code don't really care how sidedata support is added, but it needs to know if it is present. To achieve this. we use the `repo.features` attributes with a new dedicated features". Having such centralised information is more robust and will help us to remove the sidedata requirements in a later changesets. Differential Revision: https://phab.mercurial-scm.org/D10617

statichttp: add the missing `features` attribute This make statichttp closer to the interface it is "supposed" to follow. Differential Revision: https://phab.mercurial-scm.org/D10616

sidedata: use revlogv2 requirement in the test helper We are about to drop the requirement dedicated to sidedata (since revlogv2 will support them unconditionally and previous version will not.). To prepare this more we adapt the test code to not use that requirements. Differential Revision: https://phab.mercurial-scm.org/D10615

upgrade: display sidedata before upgrade This is a small gratuitous change that help making the test simpler to understand on its own. Differential Revision: https://phab.mercurial-scm.org/D10614

revlog: determine sidedata support based on the revlog version Revlog version "2" support sidedata, previous version does not. So lets make the initialization simpler. Differential Revision: https://phab.mercurial-scm.org/D10613

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