debugcommands: introduce a debug command to repair repos affected by issue6528 This command is quite basic and slow, it will loop over the entirety of the filelogs in the repository and check each revision for corruption, then fixes the affected filelogs. It takes under 25 minutes for Mozilla-Central on my not-top-of-the-line laptop, using the `--to-report` and `--from-report` options will make this pretty tolerable to use, I think. This change also introduces a test for the fix. Differential Revision: https://phab.mercurial-scm.org/D11239

contrib: switch the Windows bootstrap environment to py3.9 Use the built in `venv` module instead of `virtualenv` for simplicity, and upgrade to a modern Mercurial that supports py3. One issue here is that `venv` doesn't copy `python3{,Y}.dll` into the `Scripts` subdirectory, so running the `hg.exe` that gets installed immediately fails on a clean system because Python isn't in `PATH`. There is code in `python.exe` to detect when it is in a venv and add the original python install to the DLL lookup path, which we don't do in `hg.exe` yet. The simple workaround for now is to run the `hg` script with `python.exe`. Typically `PYTHONLEGACYWINDOWSSTDIO` must be set in the environment on Windows, but the clone process works without it. Differential Revision: https://phab.mercurial-scm.org/D11275