Cannot open `directory` because it is a submodule'

Hi,

I'm having an issue with Git submodules as described below:

## Description

When a directory is a GIT submodule, an attempt to open it with the `@jupyterlab/github` extension
displays an error:

## Reproduce
1. Create a GIT submodule on the file system typing something like this on the
command line
~~~ bash
# To add and clone a new submodule
$ git submodule add https://github.com/chaconinc/DbConnector
$ git clone https://github.com/chaconinc/MainProject
$ git submodule init
$ git submodule update
~~~
2. Open _Jupyter Lab_ with the `@jupyterlab/github` extension installed.
3. Open the GIT panels and browse to the submodule
4. See error 'Cannot open `` because it is a submodule'

## Expected behavior

Git submodules should be supported like regular Git modules. That would be extremely helpful!

## Context

- Python package version:

# packages in environment at c:\Miniconda:
#
# Name Version Build Channel
jupyterlab-github 1.0.0 pypi_0 pypi

Thanks,

WetHat