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