RFR: Only consider resolved backports when applying sync label

[Robin Westberg]

Issues that are still open should not be considered when calculating which issues should have the sync label applied.

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Commit messages:
 - Only consider resolved issues when labelling backports

Changes: https://git.openjdk.java.net/skara/pull/952/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=952&range=00
  Stats: 33 lines in 2 files changed: 30 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/952.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/952/head:pull/952

PR: https://git.openjdk.java.net/skara/pull/952