RFR: 1072: Skara should validate the commit hash of a Backport PR [v3]

[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.

Erik Joelsson has updated the pull request incrementally with one additional commit since the last revision:

  Add test

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Changes:
  - all: https://git.openjdk.java.net/skara/pull/1184/files
  - new: https://git.openjdk.java.net/skara/pull/1184/files/f0cfcfff..bb129b9e

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=1184&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=1184&range=01-02

  Stats: 72 lines in 1 file changed: 72 ins; 0 del; 0 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