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