Track Gemfile and Gemfile.lock

Forgot to do this in the previous patch.

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
Shift `colorized_line_truncate_to_fit` to GitUtils module

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
Add tests for GitUtils::colorized_line_truncate_to_fit

Test framework of choice is RSpec.

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>