Restore Diff level working-tree change

- Add a dedicated native Diff fixture that commits baseline app.rb content and leaves an unstaged line-26 change from data.json to server.json.
- Update the Diff level setup metadata so app.rb is no longer represented as an empty file.
- Add a native runtime regression for `git diff` showing the expected line-26 hunk.

app/build.gradle.kts

@@ -19,8 +19,8 @@ android {
         applicationId = "solutions.tretter.githugandroid"
         minSdk = 26
         targetSdk = 35
-        versionCode = 126
-        versionName = "0.1.125"
+        versionCode = 127
+        versionName = "0.1.126"
 
         testInstrumentationRunner = "androidx.test.runner.AndroidJUnitRunner"
         vectorDrawables.useSupportLibrary = true