Nimbe pulls incorrect submodule version from a non master branch
Due to some version changes in the library I'm binding it is required to have different branches with different submodule versions from the `master` branch.

`1.0` branch: https://github.com/nimgl/nimgl/tree/1.0/src/nimgl/private
`master` branch: https://github.com/nimgl/nimgl/tree/master/src/nimgl/private

They both target different cimgui commit hashes, the issue comes when a user wants to install the branch `1.0` with `nimble install nimgl@#1.0`, it correctly downloads all files that are in the repo but fails at correctly updating the submodule, instead it just keeps the version that is referenced in the `master` branch which causes a SEGFAULT when trying to reference incorrect data: https://github.com/nimgl/nimgl/issues/25.
My naive idea of the error would be that doing the same with just the git cli actually requires an extra command: `git pull --recurse-submodules` after checkout the desired branch, which nimble lacks. 

This issue is not happening in a "mirror" repo that is kept as an independent package: https://github.com/nimgl/imgui it correctly downloads the desired submodule version.