RFR: forge: use correct endpoints for commits in GitLab

[Erik Helin]

Hi all,

please review this patch that uses the correct REST API endpoint for commits for GitLab.

Testing:
- [x] Manual verification of endpoints
- [x] `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - forge: use correct endpoints for commits in GitLab

Changes: https://git.openjdk.java.net/skara/pull/671/files
 Webrev: https://webrevs.openjdk.java.net/skara/671/webrev.00
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.java.net/skara/pull/671.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/671/head:pull/671

PR: https://git.openjdk.java.net/skara/pull/671