AUTHORS.in tracks the maintainers, as well as some folks who were
previously in AUTHORS but don't have a git commit with proper
attribution.
Generated output is sorted alphabetically and lacks pretty spacing, so
tweak AUTHORS.in to follow the same format.
Additionally, drop the syntax-check rule that previously validated
AUTHORS against git log.
(cherry picked from commit 7b21981cdb)
Conflicts:
.mailmap
AUTHORS
