The find_base_commit program locates the newest commit with a full set of
test results. This is a good base for local testing when comparing with the
results from the official bots, as all the needed results are there.
Change-Id: Ie06020ca98317b82f46589ad6dede37361acf744
Reviewed-on: https://dart-review.googlesource.com/c/81040
Reviewed-by: Alexander Thomas <athom@google.com>