Add script for creating an MR with many commits

What does this MR do?

Adds script for creating an MR with many commits

Also does some cleanup and refactoring

Related to gitlab-org/quality/performance#16 (closed)

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