Everything in Magento Enterprise Cloud Edition is Git-driven. Not only the code, but also the infrastructure. A project contains multiple environments, which are Git branches.
Every branch enables you to develop features, test, or fix bugs, just like you normally do in development. Not only that, every pull request to a branch is a separate staging system you can view in a browser and test using command-line or other automated test tools.
You can test every feature in isolation and put it into production with a single click, or a single command on the command-line.
The following figure shows how it works at a high level.
You get Magento code from our repository, create a project which is based in Git, then do your customization in an integration system. You can use the Magento Cloud command line to configure your project and use an automated deployment process to push code from integration to staging and finally to production.
Use any methodology
You can use any methodology you already have or you can invent new ones. For example, you can use a Development -> Staging -> Production workflow.
As another example, if you are using an agile methodology such as scrum you can create branches for every sprint.
From each sprint, you can have branches for every user story. All the stories become testable. You can continually merge to the sprint branch and validate that on a continuous basis. When the sprint ends, there is no testing bottleneck, and you can just merge to master and put the whole sprint into production.