Fix CreateBranchService after Repository#commit_file was renamed to create_file

What does this MR do?

It fixes CreateBranchService which contains references to Repository#commit_file that no longer exists. It also adds tests for CreateBranchService.

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #29687 (closed)