Use proper Git repository for new branch tests

[geofflamrock]

This PR is part of a series that improves tests by using a real Git repository, not a substituted one:

• Use proper Git repository for add branch tests #144
• Use proper Git repository for new branch tests #145
• Use proper Git repository for cleanup stack tests #146
• Use proper Git repository for delete stack tests #147
• Use proper Git repository for new stack tests #148
• Use proper Git repository for remove branch tests #153
• Use proper Git repository for stack switch tests #154
• Use proper Git repository for stack status tests #155
• Use proper Git repository for update stack tests #156
• Use proper Git repository for create pull requests tests #157
• Use proper Git repository for open pull requests tests #158

This PR changes tests for the branch new command to use a proper Git repo.