RFR: 441: Proper backport support

[Robin Westberg]

Hi all,

Please review this change that improves the issue updater to provide backport support similar to the hgupdater.

Best regards,
Robin

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Commit messages:
 - Initial implementation

Changes: https://git.openjdk.java.net/skara/pull/685/files
 Webrev: https://webrevs.openjdk.java.net/skara/685/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-441
  Stats: 1756 lines in 10 files changed: 1736 ins; 13 del; 7 mod
  Patch: https://git.openjdk.java.net/skara/pull/685.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/685/head:pull/685

PR: https://git.openjdk.java.net/skara/pull/685