Move repository creation outside snippet db transaction

What does this MR do?

According to !25905 (comment 299597586), we shouldn't perform any RPC git related action inside a db transaction to avoid holding a backend connection on pgbouncer.

In this MR we move the repository creation, when the snippet is created, outside the db transaction.

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