hghave: update rustfmt criterion In 9ebc10ad4a04 I updated `rustfmt` without touching hghave, which means that the CI has been skipping the format test ever since. Thankfully, only one offending line exists in the code that's been introduced since. Differential Revision: https://phab.mercurial-scm.org/D12180

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