fixup! redirect stdout to sys.stdout in command_wrappers
use tempdir in test_GitRepository.py
add expected stdout to doctests
fixup! use tempdir in test_GitRepository.py
avoid failing tests if GBP_TESTS_NOCLEAN is set
more tempdir usage in tests
remove deprecated method _git_getoutput and update all usages
cleanup tests

All tests include the context module which consolidates tmpdir creation
and cleanup, undoes a chdir in teardown and silences log messages.