RFR: bridgekeeper: support multiple close messages

[Erik Helin]

Hi all,

please review this small patch that allows the bridgekeeper bot to close pull requests with different messages
depending on the type of repository.

Testing:
- [x] `make test` passes on Linux x64
- [x] Added an additional unit test

Thanks,
Erik

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Commit messages:
 - bridgekeeper: support multiple close messages

Changes: https://git.openjdk.java.net/skara/pull/720/files
 Webrev: https://webrevs.openjdk.java.net/skara/720/webrev.00
  Stats: 82 lines in 3 files changed: 68 ins; 0 del; 14 mod
  Patch: https://git.openjdk.java.net/skara/pull/720.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/720/head:pull/720

PR: https://git.openjdk.java.net/skara/pull/720