Issue Type: Feature Request

Testing microsoft/vscode#64262.

1. check out a PR
2. make changes in your workspace
3. in the SCM viewlet choose Suggest Edit from the drop down menu
   -> Warning:

There are no staged changes for suggestions.

As when committing without staged changes we should "stage" the changes after the user confirmed.

Extension version: 0.337.0
VS Code version: Code - Insiders 1.30.0-insider (fa87e67ebfd6b8014ba697ddbc6c2d6c0e2cb970, 2018-12-04T09:26:33.257Z)
OS version: Darwin x64 18.0.0