fix: incorrect branch encoding

This MR adds tests and fixes for the following issue:

When we removed the legacy GitLab service (!428 (merged)), we introduced a bug when we escaped some API query parameters twice.

That created malformed API URLs like these ones:

- https://gitlab.example.com/api/v4/projects/5261717/merge_requests?state=opened&source_branch=feature%252F123
+ https://gitlab.example.com/api/v4/projects/5261717/merge_requests?state=opened&source_branch=feature%2F123

notice double-escaped source_branch=feature/123 parameter

- https://gitlab.example.com/api/v4/projects/group%2Fproject/repository/tree?ref=feat%252F123&path=path%252Fto%252Ffile
+ https://gitlab.example.com/api/v4/projects/group%2Fproject/repository/tree?ref=feat%2F123&path=path%2Fto%2Ffile

notice double-escaped ref=feat/123 and path=path/to/file parameters

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(The issue affected "For Current Branch" and "Remote Repository" features)

I also removed duplication from the gitlab_service.tests.ts by initializing the GitLabService in the beforeEach block.

Related to #523 (closed)