Add some more notes and ideas for the future.
Pass os.environ explicitly to the Environment constructors.

This will make it easier to test them using simulated environments.
Add a --show-env option, for testing/debugging purposes.
README: Clarify default for administrator.
Allow the GIT_CONFIG environment variable to be specified.

Add a git_config parameter to the Config constructor that allows the
GIT_CONFIG environment variable to be set during invocations of "git
config".
Fix setting of "administrator".

If the "administrator" is not configured directly, try using the
envelopesender setting before falling back to the generic default
string.
Determine GIT_DIR when it is needed, not as part of module init.

Eliminate one global variable and let it deal correctly with directory
changes.
Add tests that the environment is set up correctly.
Merge branch 'test-env'

Add some automated tests that the environment is set up correctly
based on the configuration.