plugin/GitLabPulls: support for using a personal access token (PAT)

In order to access protected or private repositories. Using the target repository URL along with the merge-request ref instead of the source repository url and branch is necessary to avoid running into issues if the source repository is not actually accessible to the user Hydra is authenticating as.

Thanks Alexei Robyn for this patch.

Created by  Antoine Eiche  on November 20, 2018
PR4W2IUBHWKQ7BRFLXELHBVRU7VOSLKO4DVZIPSHLXZ422LWP2SQC
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