Move GitOperationService to Gitlab::Git

What does this MR do?

Moves GitOperationService from app/services to lib/gitlab/git.

• rename app/services/git_operation_service.rb to lib/gitlab/git/operation_service.rb
• rename the class in the file from GitOperationService to Gitlab::Git::OperationService
• move the CommitError constant from app/models/repository.rb to lib/gitlab/git.rb

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

All code that touches Git repositories is being moved into lib/gitlab/git.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?