Fix the fetching of recent builds and revision parsing in garden-o-matic.

The code was not prepared to handle paginated build listings coming back
from google storage. This patch changes the code to figure out the most
recent build number on each builder, and then attempt to fetch only the
most recent 100 builds so that we don't fetch way more data than we need.

R=abarth@chromium.org
BUG=288967

Review URL: https://chromiumcodereview.appspot.com/23803009

git-svn-id: svn://svn.chromium.org/blink/trunk@157780 bbb929c8-8fbe-4397-9dbb-9b2b20218538