Make sure AddToGitignore keeps the newline at the end of the file (#6847)

Author: greg-at-moderne

rewrite-core/src/main/java/org/openrewrite/AddToGitignore.java

@@ -126,6 +126,7 @@ public PlainText visitText(PlainText text, ExecutionContext ctx) {
 
             private String mergeGitignoreEntries(String existing, String newEntries) {
                 String separator = existing.contains("\r\n") ? "\r\n" : "\n";
+                boolean endsWithNewline = existing.isEmpty() || existing.endsWith("\n");
 
                 Set<String> existingRules = new LinkedHashSet<>();
                 Set<String> existingWildcardPatterns = new LinkedHashSet<>();
@@ -181,7 +182,8 @@ private String mergeGitignoreEntries(String existing, String newEntries) {
 
                 result.addAll(linesToAdd);
 
-                return String.join(separator, result);
+                String joined = String.join(separator, result);
+                return endsWithNewline ? joined + separator : joined;
             }
 
             private String normalizeRule(String rule) {

rewrite-core/src/test/java/org/openrewrite/AddToGitignoreTest.java

@@ -447,6 +447,30 @@ void doNotAddSuperfluousEntries() {
         );
     }
 
+    @Test
+    void addEntryToFileEndingWithNewline() {
+        rewriteRun(
+          spec -> spec.recipe(new AddToGitignore("*.log", null)),
+          text(
+            "*.tmp\n.DS_Store\n",
+            "*.tmp\n.DS_Store\n\n*.log\n",
+            spec -> spec.path(".gitignore").noTrim()
+          )
+        );
+    }
+
+    @Test
+    void addEntryToFileEndingWithWindowsNewline() {
+        rewriteRun(
+          spec -> spec.recipe(new AddToGitignore("*.log", null)),
+          text(
+            "*.tmp\r\n.DS_Store\r\n",
+            "*.tmp\r\n.DS_Store\r\n\r\n*.log\r\n",
+            spec -> spec.path(".gitignore").noTrim()
+          )
+        );
+    }
+
     @Test
     void doNotAddSuperfluousEntriesWithMultiplePatterns() {
         rewriteRun(