Update Get Single Repository Branch information to include ability to use `ref=`

What does this MR do?

You can use ref= when targeting project repository branch queries. Eg:

https://gitlab.com/api/v4/projects/:id/repository/branches/?ref=<branch ref number or name>

This is particularly useful when using Advanced Group/Global Search targeting blobs. The data returned caries the ref of the branch, not the name.

Update to add to the Parameters

Screenshots

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

Documentation change. Minimal risk.

• 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