[skara] RFR: Allow issues to be excluded from the hgupdate-sync logic

[Robin Westberg]

Allow manual exclusion of certain issues when evaluating which ones should be marked with the hgupdate-sync label.

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Commit messages:
 - Allow issues to be excluded from the hgupdate-sync logic

Changes: https://git.openjdk.java.net/skara/pull/1037/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1037&range=00
  Stats: 86 lines in 2 files changed: 82 ins; 1 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/1037.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1037/head:pull/1037

PR: https://git.openjdk.java.net/skara/pull/1037