RFR: 1611: Improve adding and removing of labels in GitLab

[Zhao Song]

Improved adding and removing of labels in GitLabMergeRequest.

And make the label handling consistent with GitHubPullRequest.

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Commit messages:
 - improved adding and removing of labels in GitLabMergeRequest

Changes: https://git.openjdk.org/skara/pull/1396/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1396&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-1611
  Stats: 22 lines in 1 file changed: 6 ins; 12 del; 4 mod
  Patch: https://git.openjdk.org/skara/pull/1396.diff
  Fetch: git fetch https://git.openjdk.org/skara pull/1396/head:pull/1396

PR: https://git.openjdk.org/skara/pull/1396