• VSCode Version: 1.30.2
• OS Version: macOS 10.14.2

Steps to Reproduce:

1. Compile and run the virtual-document-sample.
2. Extension host window -> Explorer view
3. Ensure you have at least one "untitled" document open in "Open Editors"
4. Create a cowsay virtual document: > cowsay
5. Right-click the virtual document

Actual: the context menu for the virtual doc does not include the compare commands "Select for compare" and "Compare with selected". The context menu for the the untitled doc does.

Expected: context menu for virtual docs should include the compare commands.

Note: the hard part already seems to be implemented, can already compare a virtual doc with another doc by selecting the other doc first! But this workaround only works when a non-virtual document is present. Two virtual docs ought to be comparable in the same way. Just need to enable the commands.