Submodules & subtrees
Submodules
Submodules are listed in the sidebar. A submodule's context menu offers Init & Update, Update to the commit the parent records, Sync URL and Open as Repository.
Adding or removing a submodule is not built yet; use git submodule for that.
Subtrees
The + in the sidebar adds a subtree: a prefix (the folder), a repository, a branch, and whether to squash its history. A subtree's context menu offers Pull and Push.
Subtrees need git subtree, which some Git packages ship separately.