We need to modify jcheck to support a "start" date from which changesets are checked.

The start date is an optional config parameter. If set, then changesets before that date are not checked (they always pass unless they are in the blacklist). This is needed because our history has many thousands of changesets that are non-compliant (most of the recent ones are, but we can't enforce that until we enable jcheck).

Webrev: http://cr.openjdk.java.net/~kcr/jcheck/start-date/webrev.01/