I keep losing this utility, used as part of other tools to auto-update
the AUTHORS and CONTRIBUTORS files. Check it in to the repo so I
don't lose it, and so others can use it as well.
Updates #12042
Change-Id: Ib5886b85799087aaaddcec4c81169e2726322c05
Reviewed-on: https://go-review.googlesource.com/16824
Reviewed-by: Russ Cox <rsc@golang.org>
