rhg: Fall back to Python for unsupported revset syntax rhg only supports a small subset of the syntax. On parse error, this gives Python a chance instead of aborting immediately. Differential Revision: https://phab.mercurial-scm.org/D10097

tests: Add `rhg` and `no-rhg` for #require and #if in .t files With this we can allow some small differences in expected output between Rust-based and Python-based code paths. Differential Revision: https://phab.mercurial-scm.org/D10096

tests: Enable rhg fallback to Python by default in tests This premise of `run-tests.py --rhg`: fallback should make `rhg` behave the same as `hg`, except faster in some cases. To test run the whole test suite with installed `rhg` as `hg` and with fallback enabled. Differential Revision: https://phab.mercurial-scm.org/D10187

rhg: Add environment variables for fallback configuration For the `rust-tests.py --rhg` we want every `hg` command in tests to run `rhg` with fallback enabled, but other methods of setting configuration are limited or disruptive as discussed in code comment. Differential Revision: https://phab.mercurial-scm.org/D10186

paths: add a `*` special path to define default sub option Differential Revision: https://phab.mercurial-scm.org/D10163

ui: pass a `ui` object to `paths.getpath` I want to introduce more path's suboption and make it possible to use default value for them. Processing theses sub-options might result in warnings. We need a `ui` object to issue such warnings. To make things simpler, we add an helper method on the `ui` object. Differential Revision: https://phab.mercurial-scm.org/D10162

configitems: add TODOs blocking the move out of experimental for revlogv2 These are the todos so far, we probably will have more, but we might as well list them while they're fresh in our minds. Differential Revision: https://phab.mercurial-scm.org/D10216

sidedata-exchange: rewrite sidedata on-the-fly whenever possible When a A exchanges with B, the difference of their supported sidedata categories is made, and the responsibility is always with the client to generated it: - If A pushes to B and B requires category `foo` that A does not have, A will need to generate it when sending it to B. - If A pulls from B and A needs category `foo`, it will generate `foo` before the end of the transaction. - Any category that is not required is removed. If peers are not compatible, abort. It is forbidden to rewrite sidedata for a rev that already has sidedata, since that would introduce unreachable (garbage) data in the data file, something we're not prepared for yet. Differential Revision: https://phab.mercurial-scm.org/D10032