RFR: pr: add "backport" label for backports

[Erik Helin]

Hi all,

please review this patch that adds the label "backport" for backport-style pull requests.

Testing:
- [x] Updated a number of unit tests

Thanks,
Erik

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Commit messages:
 - Updates
 - pr: add "backport" label for backports

Changes: https://git.openjdk.java.net/skara/pull/923/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=923&range=00
  Stats: 11 lines in 2 files changed: 11 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/923.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/923/head:pull/923

PR: https://git.openjdk.java.net/skara/pull/923