Replace gem Rugged with Git

[mxey]

Overview

Rugged incurs a very high cost at install time due to the compilation of libgit2. It takes way too long and can have annoying errors.

octocatalog-diff requires almost nothing from Git, therefore use a much lighter weight approach and shell out to Git with a different gem.

I understand octocatalog-diff is apparently unmaintained, I am just parking this here in case other people are interested.

Checklist

• [ ] Make sure that all of the tests pass, and fix any that don't. Just run rake in your checkout directory, or review the CI job triggered whenever you push to a pull request.
• [ ] Make sure that there is 100% test coverage by running rake coverage:spec or ignoring untestable sections of code with # :nocov comments. If you need help getting to 100% coverage please ask; however, don't just submit code with no tests.
• [x] If you have added a new command line option, we would greatly appreciate a corresponding integration test that exercises it from start to finish. This is optional but recommended.
• [x] If you have added any new gem dependencies, make sure those gems are licensed under the MIT or Apache 2.0 license. We cannot add any dependencies on gems licensed under GPL.
• [ ] If you have added any new gem dependencies, make sure you've checked in a copy of the .gem file into the vendor/cache directory.

/cc [related issues] [teams and individuals, making sure to mention why you're CC-ing them]