RFR: 1072: Skara should validate the commit hash of a Backport PR

[Erik Joelsson]

Add a check for "Backport <hash>" titles that the given hash isn't the head of the PR itself. If the user mistakenly uses that hash, Skara is very likely to get fooled into thinking the backport is clean.

While investigating this, I also discovered that the CheckWorkItem was inconsistent with not repeating "backport" errors, so I implemented a general mechanism for this, which guarantees that the same exact error message is never repeated. I believe this will be good enough, and certainly better than the existing behavior.

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Commit messages:
 - SKARA-1072

Changes: https://git.openjdk.java.net/skara/pull/1184/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1184&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1072
  Stats: 102 lines in 2 files changed: 89 ins; 7 del; 6 mod
  Patch: https://git.openjdk.java.net/skara/pull/1184.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1184/head:pull/1184

PR: https://git.openjdk.java.net/skara/pull/1184