Add a new module: viewer.utils.
test_utils: Remove spec_set from mock.patch('os.chdir').

As os.chdir identifies itself as a bultin function, the speccing does not work. Therefore, the spec_set argument is useless in this case.
Add a base of a new module: viewer.git.
test_git: Move the patches before the testing class.
test_git: Fix a path.
git: Raise exception when the git command is not found.