MailingListNotifier will not send an email notification when the first branch of some repos is created

A Leyden committer created a new branch (pregenerate-lambdas) and pushed to it, but skara bot didn't sent email notification to leyden-dev@openjdk.org.

After investigating, I found the reason is that in the configuration of NotifyBot, we configured branches as '!master'. Due to this configuration, 'master' branch would be ignored in knownRefs. Therefore, in method RepositoryWorkItem#handleNewRef, The NotifyBot would not be able to find a candidate and it will return immediately.