RFR: 542: Adjust label (cc) command logic

[Robin Westberg]

Hi all,

Please review this change that adjusts how the `/labels` (or `/cc`) command works, details in the issue.

Best regards,
Robin

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Commit messages:
 - Initial implementation

Changes: https://git.openjdk.java.net/skara/pull/749/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=749&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-542
  Stats: 210 lines in 7 files changed: 169 ins; 19 del; 22 mod
  Patch: https://git.openjdk.java.net/skara/pull/749.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/749/head:pull/749

PR: https://git.openjdk.java.net/skara/pull/749