RFR: Ensure that the pr head is present locally when pushing a dependency branch

[Robin Westberg]

When the pull request branch notifier creates a dependency target branch, we need to ensure that the head commit is actually present locally.

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Commit messages:
 - Ensure that the pr head is present locally

Changes: https://git.openjdk.java.net/skara/pull/918/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=918&range=00
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/918.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/918/head:pull/918

PR: https://git.openjdk.java.net/skara/pull/918