RFR: backports: handle original commit without issues

[Erik Helin]

Hi all,

please review this patch that handles backports of commits that are missing proper references to issues in JBS.

Testing:
- [x] Added a new unit test

Thanks,
Erik

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Commit messages:
 - backports: handle original commit without issues

Changes: https://git.openjdk.java.net/skara/pull/986/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=986&range=00
  Stats: 83 lines in 2 files changed: 68 ins; 4 del; 11 mod
  Patch: https://git.openjdk.java.net/skara/pull/986.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/986/head:pull/986

PR: https://git.openjdk.java.net/skara/pull/986