RFR: 1611: Improve adding and removing of labels in GitLab [v2]

[Zhao Song]

> Improved adding and removing of labels in GitLabMergeRequest.
> 
> And make the label handling consistent with GitHubPullRequest.

Zhao Song has updated the pull request incrementally with one additional commit since the last revision:

  Add testLabels in GitLabRestApiTest

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Changes:
  - all: https://git.openjdk.org/skara/pull/1396/files
  - new: https://git.openjdk.org/skara/pull/1396/files/380b021d..e58767a9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=skara&pr=1396&range=01
 - incr: https://webrevs.openjdk.org/?repo=skara&pr=1396&range=00-01

  Stats: 43 lines in 2 files changed: 36 ins; 6 del; 1 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