Path incorrectly copied to tasks.json

_From @drazen64 on January 13, 2018 17:15_

- VSCode Version: 1.19.2
- OS Version: WIndows 10 Enterprise, v1709, build 16299.192

Steps to Reproduce:

1. Create new project `dotnet new xunit --name WebServer.Test`
2. Go to project folder and add reference to the existing project
`dotnet add reference ..\Webserver\Webserver.csproj`
This adds following lines to the csproj file:
```

```

3. Start VS Code and click "Yes" when it warns that required assets are missing.
4. Path to the referenced project in tasks.json is missing `../` at the beginning
```
{
"version": "2.0.0",
"tasks": [
{
"taskName": "build",
"command": "dotnet",
"type": "process",
"args": [
"build",
"Webserver/Webserver.csproj"
],
"problemMatcher": "$msCompile"
}
]
}
```

Does this issue occur when all extensions are disabled?: VS Code does not offer to create tasks.json

_Copied from original issue: Microsoft/vscode#41575_