RFR: forge: use correct endpoint for GitLab branches

[Erik Helin]

Hi all,

please review this small patch that fixes the "branches" endpoint for
`GitLabRepository` (it is `/repository/branches`, not `/branches`).

Testing:
- `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - forge: use correct endpoint for GitLab branches

Changes: https://git.openjdk.java.net/skara/pull/572/files
 Webrev: https://webrevs.openjdk.java.net/skara/572/webrev.00
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/572.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/572/head:pull/572

PR: https://git.openjdk.java.net/skara/pull/572