Extend `MergeToRefService` to create merge ref from an arbitrary ref

What does this MR do?

• CE: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30361
• EE: Here

(There is a CE port, but technically all the changes are made in CE, so there are no need to create EE MR, but it's good measurement to check the pipeline passes on EE side)

Currently, MergeToRefService is for creating merge ref from source sha (HEAD of the source branch of merge request) and target branch of the merge request. In order to create refs/merge for https://gitlab.com/gitlab-org/gitlab-ee/issues/11222, we need to make this service to be able to generate merge ref from source sha and the other refs/merge.

• Issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/11222
• Gitaly: gitaly!1210 (merged)
• Related: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14305

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Performance 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

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