Test for #18615

• Windows @sandy081
• OS X @ramya-rao-a
• Linux @Tyriar

Complexity: 5

Make sure you Enable SCM Preview in the command palette. Most of this code is new, make sure you give a it proper smoke test:

• Git output channel
• Dirty diff (gutter decorations)
• SCM viewlet
  • Click, keyboard nav
  • Title, inline, context menu actions
  • Commit box
• Git status bar
• Merge conflict decorations
• Fetch happens automatically, unless configured otherwise
• SCM Viewlet shows progress during operations
• Global commands
• Editor title actions
• No repo workspace shows "Git Init" action
• Scoped git workspace: open a folder within a git repo, everything should work
• Editor selections
  • Staging, unstaging (global commands)
  • Reverting (global command)
• If you set a git commit template it is picked as a default value for the commit box
• If you use a repository that requires authentication, you get prompted for credentials
• Clone global command