vscode.diff only allows selecting language for right-hand diff

[joeskeen]

• VSCode Version: 1.17.0-insider commit 06529db
• OS Version: Windows 10 Pro v1703

Steps to Reproduce:

1. Right-click a plaintext file and select 'select for compare'
2. Right-click another plaintext file and select 'compare with ...'
   (The language in the bottom-right should be "Plain Text")
3. Click "Plain Text" and select another language
   You will see syntax highlighting for the right-hand diff, but the left-hand diff remains plain text

Reproduces without extensions: Yes