Issue Type: Feature Request

Calling cat /some/file | code --diff /some/other/file - simply opens /some/other/file instead of generating a diff between its content and stdin. Obviously one would not cat the stdin but the point still holds and generating an intermediate file is not always desired nor possible.

VS Code version: Code 1.51.0 (fcac248, 2020-11-05T18:16:10.374Z)
OS version: Linux x64 5.8.0-26-generic