Refactor test_repository.py and impact_git_test.py

[RomainLefeuvre]

Refactoring – test_repository.py

• Make the repo generation cleaner, remove unnecessary branch that were generated
• Use protobuf type for events

Refactoring - impact_git_test.py

• Move to a template architecture
• Remove inconsistent tests
• Support cherry picking test

Note :
impact.py has been modified to handle repository that do not support caching. We are still trying to find a solution to enable the cache for synthetic repository created with pygit2.