UnifiedDiff for staged files with Git

The UnifiedDiff compares currently against a file from the workspace. As soon as a file is staged, the comparison falls back to the old compare editor as now two Git states (index vs head) are compared.

The fallback is decided in  CompareUIPlugin.prepareUnifiedDiff(...) . The unified diff needs a workspace file ( IFileEditorInput ) to overlay onto:
````
if (leftEditorInput instanceof IFileEditorInput) { // working-tree side
return new UnifiedDiffSource(..., UnifiedDiffMode.REVERT_MODE, ...);
}
if (rightEditorInput instanceof IFileEditorInput) {
return new UnifiedDiffSource(..., UnifiedDiffMode.OVERLAY_READ_ONLY_MODE, ...);
}
return null; // ← neither side is a workspace file → classic compare editor
````

• Unstaged changes (working tree vs. index/HEAD): the left side is the workspace file → unified diff works. ✅
• Staged changes (index vs. HEAD): both sides are Git revisions, neither is an  IFileEditorInput  →  prepareUnifiedDiff  returns  null  → classic compare. ❌

Not sure how we should handle that. Should we still compare against the workspace file? Or leave that to the old compare?

cc @tobiasmelcher

Start in CompareUIPlugin.prepareUnifiedDiff(...) and trace how IFileEditorInput, UnifiedDiffSource, and the fallback compare editor are selected. Compare the staged index-versus-HEAD path with the unstaged workspace-file path, then determine and document the expected unified-diff behavior for staged files. Done means the chosen behavior is implemented and covered by the relevant comparison checks.

Written by the indexing model from the issue text.