[Dev] Add infrastructure for testing Git interactions in merge_arrow_pr.py

Currently, there are no tests in 'test_merge_arrow_pr.py' for functions in 'merge_arrow_pr.py' that require interaction with Git. 

Adding infrastructure to allow interactions with a test Git repository or mocked environment will allow 'test_merge_arrow_pr.py' to qualify steps that use Git commands.

**Reporter**: [Fiona La](https://issues.apache.org/jira/browse/ARROW-16050) / @lafiona

_{**Note**: *This issue was originally created as [ARROW-16050](https://issues.apache.org/jira/browse/ARROW-16050). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start by reading merge_arrow_pr.py and the existing tests in test_merge_arrow_pr.py to identify functions that invoke Git commands. Run the current test file, then add reusable infrastructure for a test Git repository or mocked Git environment. Done means Git-dependent steps in merge_arrow_pr.py are covered by qualifying tests in test_merge_arrow_pr.py.

Written by the indexing model from the issue text.