Remove the following line form the test_helper.rb:

# Silence deprecation warnings during tests
Git::Deprecation.behavior = :silence

and silence all the deprecation warnings.

Avoid deprecation warnings by:

1. Not having tests call deprecated methods (or in deprecated ways)
2. Not having the lib code call deprecated methods

For each deprecation, add a test that ensures that the deprecation message is triggered under the right circumstances.

Finally, once all deprecation warnings are silenced, make it so that during tests, depreciation warnings cause the test to fail.

This issue was partially dealt with in PR #826.