Revert "remove diff classifier"

This reverts commit ae6db478c5a22c7cb9ff9603f01bd88afdf4f67b.
Remove unnecessary reference to AvalonEdit assembly
Fix background color for diff panel in Pending Changes Window
Clear the diff panel in the Pending Changes window when no files are selected
Prohibit user input in the diff editor in the Pending Changes window
Hide unnecessary margins in the diff editor in the Pending Changes window
Normalize line endings in the file produced by GitFileStatusTracker.DiffFile
Merge branch 'lineends' of https://github.com/sharwell/Git-Source-Control-Provider