RFR: 1449: Changing the original hash in a backport PR doesn't work

[Erik Joelsson]

If a user tries to change the original hash for a backport PR, by changing the title to "Backport HASH", it appears to work, but in reality, several things behind the scenes do not. The logic for finding the original commit will always just return the first hash that was set and ignores any updates/changes. This affects the clean evaluation and the commit message.

This patch changes the method for finding the original hash so that the last one is returned instead. A new test is added to verify this functionality, both for the clean evaluation as well as the final commit message.

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Commit messages:
 - SKARA-1449

Changes: https://git.openjdk.java.net/skara/pull/1321/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1321&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1449
  Stats: 135 lines in 2 files changed: 132 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/1321.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1321/head:pull/1321

PR: https://git.openjdk.java.net/skara/pull/1321