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/src/main/java/solutions/tretter/githugandroid/NativeLevelFixtures.kt

@@ -12,6 +12,7 @@ internal fun materializeNativeLevelFixture(
         "branch_at" -> fixture.branchAt()
         "checkout_tag" -> fixture.checkoutTag()
         "checkout_tag_over_branch" -> fixture.checkoutTagOverBranch()
+        "diff" -> fixture.diff()
         "fetch" -> fixture.fetch()
         "pull" -> fixture.pull()
         "push_branch" -> fixture.pushBranch()
@@ -176,6 +177,14 @@ private class NativeLevelFixture(
         return true
     }
 
+    fun diff(): Boolean {
+        resetFiles()
+        write("app.rb", diffLevelBaselineAppRb())
+        addCommit("Add app routes", "app.rb")
+        write("app.rb", diffLevelModifiedAppRb())
+        return true
+    }
+
     fun pull(): Boolean {
         resetFiles()
         write("local_file")
@@ -387,3 +396,39 @@ private class NativeLevelFixture(
         return true
     }
 }
+
+internal fun diffLevelBaselineAppRb(): String = buildString {
+    appendLine("require 'sinatra'")
+    appendLine("require 'json'")
+    appendLine()
+    appendLine("helpers do")
+    appendLine("  def get_response(source)")
+    appendLine("    JSON.parse(File.read(source))['message']")
+    appendLine("  end")
+    appendLine("end")
+    appendLine()
+    appendLine("get '/' do")
+    appendLine("  @message = 'hello'")
+    appendLine("  erb :index")
+    appendLine("end")
+    appendLine()
+    appendLine("get '/page' do")
+    appendLine("  @message = 'page'")
+    appendLine("  erb :page")
+    appendLine("end")
+    appendLine()
+    appendLine("get '/yet_another' do")
+    appendLine("  @message = 'another'")
+    appendLine("  erb :success")
+    appendLine("end")
+    appendLine()
+    appendLine("get '/another_page' do")
+    appendLine("  @message = get_response('data.json')")
+    appendLine("  erb :another")
+    appendLine("end")
+    appendLine()
+    appendLine("# end of application")
+}
+
+internal fun diffLevelModifiedAppRb(): String =
+    diffLevelBaselineAppRb().replace("get_response('data.json')", "get_response('server.json')")

app/src/main/java/solutions/tretter/githugandroid/levels/DiffLevel.kt

@@ -13,7 +13,14 @@ internal fun diffLevel(): Level = level(
     description = "Since your last commit, file `app.rb` was modified. Find out which line has changed.",
     hints = listOf("You are looking for the difference since your last commit."),
     commandSuggestions = listOf("git diff"),
-    setup = { RepoState(initialized = true, files = listOf(GitFile("app.rb", tracked = true)), branches = mapOf("master" to 1)) },
+    setup = {
+        RepoState(
+            initialized = true,
+            files = listOf(GitFile("app.rb", diffLevelModifiedAppRb(), tracked = true)),
+            commits = listOf(CommitNode("0000001", "Add app routes")),
+            branches = mapOf("master" to 1),
+        )
+    },
     validator = commandAnswer("26"),
     testCases = listOf(
         levelTestCase("answer changed line", "26"),