RFR: 391: Abort merge if source branch merge isn't fast-forward

[Robin Westberg]

Hi all,

Please review this change that ensures that the merge bot's local storage has been cleanly updated with the latest
destination changes (through a fast-forward merge). Otherwise, it's possible for an error to be detected when doing the
actual merge, even though it should succeed.

Best regards,
Robin

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Commit messages:
 - Mergebot updating of local storage should always be fast-forward

Changes: https://git.openjdk.java.net/skara/pull/622/files
 Webrev: https://webrevs.openjdk.java.net/skara/622/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-391
  Stats: 92 lines in 2 files changed: 81 ins; 8 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/622.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/622/head:pull/622

PR: https://git.openjdk.java.net/skara/pull/622