Use create_branch instead of fetch_source_branch when importing MRs

What does this MR do?

A minor cleanup I noticed while investigating #12643 (closed)

This is a no-op in practice; if the SHA already exists in the target repository, Gitaly skips the "fetch the ref from the source repository" logic and does a write_ref(sha, path) call: https://gitlab.com/gitlab-org/gitlab/blob/master/ruby/lib/gitlab/git/repository.rb#L512 .

Since in this call, the source repository is the same as the target repository, we get absolutely no benefit from using fetch_source_branch here.

It's good to express our intentions as directly as possible, though.

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

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers