RFR: forge: diff for GitLab commits can be empty

[Erik Helin]

Hi all,

please review this patch that handles an edge case when the `"diff"` for a commit from GitLab can be empty (the case for a renamed file for example).

Thanks,
Erik

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Commit messages:
 - forge: diff for GitLab commits can be empty

Changes: https://git.openjdk.java.net/skara/pull/932/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=932&range=00
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/932.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/932/head:pull/932

PR: https://git.openjdk.java.net/skara/pull/932