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
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true