Add some extra shortcuts

- `source`, to reload the doskey macro file
- `gbp`, to show unmerged git branches
- `gdc`, to diff the currently staged changeset
- `gdh`, to show the differences between the current state and HEAD
- Stop modifying the current PowerShell security policy; just alter it for
  the one script execution
- Improve logic in setup.bat; now only pauses if run from Explorer