Fix up for the GitDiffService (#2116)

* initial pass at untracked file changes, testing next

* add final trailing \n

* unit tests for function

* fix test execution

* tweak some comments and formattting

* another comment tweak after testing

* small change to handling fully empty files

* copilot PR feedback

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Co-authored-by: Ian Huff <ianhuff@Ians-MacBook-Pro-2.local>
Co-authored-by: Ian Huff <ianhuff@Mac.home>