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

[Zhao Song]

On Wed, 12 Oct 2022 21:36:21 GMT, Erik Joelsson <erikj at openjdk.org> wrote:

> When changing API calls like this, they need to be tested. The normal automatic tests do not cover it. See GitLabRestApiTest and add a test there to verify that labels can be added/removed/set with the new implementation. You will need to provide credentials in a special properties file (that should NOT be committed). You can generate an access token for your own user to test this out against a personal repository.

Before creating this MR, I tested it with my own repo and I think it works well. I will add tests in GitLabRestApiTest.

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