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

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