tests: Disable for rhg remaining tests that fail in that mode These cases are in features not yet implemented by rhg for which triggering a fallback is not practical. Disabling some tests allows us to reach passing CI and catch any future regression in the rest of the tests. Differential Revision: https://phab.mercurial-scm.org/D10190

rhg: Remove `rhg.fallback-executable=hg` default configuration When `rhg.on-unsupported` is configured to `fallback` and an unsupported feature is encountered, the previous default was to look for an `hg` executable in `$PATH`. This default was fragile since it was easy to end up accidentally using an older version of Mercurial installed system-wide, when a local (perhaps patched) installation was intended. Instead, it is now an error to have `rhg.on-unsupported=fallback` without also configuring an explicit path or the fallback executable. Differential Revision: https://phab.mercurial-scm.org/D10189