This change implements most of the corresponding proposal as discussed at the
4.4 and 4.6 sprints: https://www.mercurial-scm.org/wiki/AutomaticFormattingPlan
This change notably does not include parallel execution of the formatter/fixer
tools. It does allow for implementing that without affecting other areas of the
I believe the test coverage to be good, but this is a hotbed of corner cases.