Remove duplicates from related_commit_sha query

What does this MR do?

Contributes to #21686 (closed)

Problem

There is an edge case when the same merge request can be returned multiple times. If the project has always squash option on, then we create a squash commit even for merge requests with only one commit. In this case, the commit from the merge request is a squash commit at the same time.

Solution

Use UNION instead of UNION ALL to remove duplicated records.

Screenshots (strongly suggested)

Before	After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team