If you are looking for a git client that works cross-platform we recommend to use Fork.
To draw diagrams for your project or for blueprints in devonfw, we recommend the following cross-plattform tools:
ObjectAid is a nice and easy to use eclipse plugin that you can use to quickly create UML diagrams from existing code. While class-diagrams are supported for free, you need to buy a license if you want to use the other diagram types.
PlantUML is a great tool that can render UML diagrams from simple markup that can be easily managed in
gitor other version-control systems together with your code. Its simplicity allows branching and merging unlike other greedy binary UML data-formats.