RFR: 1363: update-sync incorrect sync label

[Erik Joelsson]

In SKARA-964, we fixed the labelsync bot to only consider "Fixed" issues when calculating label assignment. That fix only considered the backport issues and not the main bug. This fix tries to address this by applying the check for isFixed to the main bug as well.

While working on this, I also decided to refactor Backports a bit and remove the isFixed method from that class and instead use the newly introduced Issue::isFixed.

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Commit messages:
 - Fix comment
 - SKARA-1363

Changes: https://git.openjdk.java.net/skara/pull/1291/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1291&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1363
  Stats: 83 lines in 4 files changed: 55 ins; 20 del; 8 mod
  Patch: https://git.openjdk.java.net/skara/pull/1291.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1291/head:pull/1291

PR: https://git.openjdk.java.net/skara/pull/1291