repair: improve performance of detection of revisions affected by
issue6528
Explanations inside the patch. I've tested this on Mozilla-Central and it's
5 times faster than the naive approach on my laptop.
Differential Revision: https://phab.mercurial-scm.org/D11262
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commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).