Add support for submodules

[BeatScherrer]

I am working in a project with multiple submodules. it woulde be nice to see the diff of the submodules in the same window since all the repo make up the project.

[sindrets]

We could probably add a --submodule command option. Then we could also do --submodule=recursive to recursively diff submodules. But it would probably require additional git jobs for each submodule.

[Weissle]

Any update on this issue? I have the same situation in which a project contains many submodules. Sometimes a new feature includes changes in different submodules. I need to review the difference between the feature branch and the main branch in the submodule.