Does not support workspace folders containing multiple repositories

Summary

If your workspace folder contains mutliple subfolders that are GitLab repositories, the extension doesn't connect to the GitLab instance

Steps to reproduce

Have a folder with two repostiries as a subfolders

workspace_folder
  ├── gitlab # repostory with gitlab-org/gitlab project
  └── gitlab-vscode-extension # repository with this project

Go to this folder and open VS Code
```
cd workspace_folder
code .
```
Try to run GitLab: Show issues assigned to me command

What is the current bug behavior?

I would expect the same behaviour that we currently support for multi-root workspaces. The extension would ask whether I want to query gitlab-org/gitlab or gitlab-org/gitlab-vscode-extension project.

What is the expected correct behavior?

The extension sees that there is no repository in workspace_folder and doesn't communicate to GitLab

Possible fixes

First fix would be to make it clearly visible where the extension gets stuck. E.g. message Current workspace workspace_folder isn't a git repository, ignoring the command

The next step could be to scan the subfolders for git repositories and then use the same behaviour we have for multi-root workspaces.

Workaround

Configure a multi-root workspace instead