Have tests set the GIT_PYTHON_TRACE by default, to ensure that on error
we have a detailed log of all git commands executed.
Default to setting basic tracing, and support passing through of the
environment variable via tox.ini so that users may control the level of
logging by setting the variable.
# disable all output from GitPython
GIT_PYTHON_TRACE= tox ...
# log all commands executed, default for tests
GIT_PYTHON_TRACE=1 tox ...
# includes stdout/stderr from GitPython in logged output
GIT_PYTHON_TRACE=full tox ...
Change-Id: I71eb8f572bc2afd70f9a44f82499565fbbd4e872
7a135a1705