RFR: 1979: MailingListNotifier will not send an email notification when the first branch of some repos is created

[Zhao Song]

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

In this patch, defaultBranch would always be included in the candidate refs.

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Commit messages:
 - SKARA-1979
 - SKARA-1979

Changes: https://git.openjdk.org/skara/pull/1539/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1539&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1979
  Stats: 17 lines in 1 file changed: 8 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/skara/pull/1539.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1539/head:pull/1539

PR: https://git.openjdk.org/skara/pull/1539