RFR: notify: add handleIntegratedPullRequest callback

[Erik Helin]

Hi all,

please review this patch that adds the `handleIntegratedPullRequest` callback to `PullRequestUpdateConsumer`. I have
also updated `IssueUpdater` to make use of this new callback, as well as updated the unit tests for `IssueUpdater`.

Testing:
- [x] `make test` passes on Linux x64
- [x] Updated two unit tests

Thanks,
Erik

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Commit messages:
 - notify: add handleIntegratedPullRequest callback

Changes: https://git.openjdk.java.net/skara/pull/643/files
 Webrev: https://webrevs.openjdk.java.net/skara/643/webrev.00
  Stats: 70 lines in 5 files changed: 25 ins; 13 del; 32 mod
  Patch: https://git.openjdk.java.net/skara/pull/643.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/643/head:pull/643

PR: https://git.openjdk.java.net/skara/pull/643