Set the SHA to be merged from push options

What does this MR do?

When we set merge_when_pipeline_succeeds in the push options, we also need to provide the sha that we'll be merging.

We take the cha from the changes list that was passed into the push options, and pass it onwards to the Create or Update service.

Closes #37061 (closed)

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