Only fetch source branch when source and target projects are different

What does this MR do?

This MR updates ci/danger-review.yml to run the git fetch command only if the source branch of an MR is not from the original project, for example, if it's from a forked project.

Related to triage-ops#1401 (closed)

Testing

• ✅git fetch command ran for source branch from the forked project:
  • Job URL: https://gitlab.com/gitlab-org/quality/triage-ops/-/jobs/5114304136#L59
• ✅ git fetch command didn't run for source branch from the original project:
  • Job URL: https://gitlab.com/gitlab-com/gl-security/security-operations/infrastructure-security/automation/Alexandria/-/jobs/5114308400#L103

Check-list

• Verify the change in all affected pipelines.