RFR: Move the PR retarget responsibility to the prbranch notifier

[Robin Westberg]

Retarget any dependent pull requests before closing the target branch in the notifier, to avoid potential races.

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Commit messages:
 - Move the PR retarget responsibility to the prbranch notifier

Changes: https://git.openjdk.java.net/skara/pull/1100/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1100&range=00
  Stats: 56 lines in 4 files changed: 51 ins; 5 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/1100.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1100/head:pull/1100

PR: https://git.openjdk.java.net/skara/pull/1100