Fix multiline copyright header rule

Author: krwq

src/Microsoft.DotNet.CodeFormatting.Tests/Rules/CopyrightHeaderRuleTests.cs

@@ -29,6 +29,25 @@ class C
 
             var expected = @"// test
 
+class C
+{
+}";
+            Verify(source, expected);
+
+        }
+
+        [Fact]
+        public void CSharpSimpleMultiline()
+        {
+            _options.CopyrightHeader = ImmutableArray.Create("test1", "test2");
+            var source = @"
+class C
+{
+}";
+
+            var expected = @"// test1
+// test2
+
 class C
 {
 }";
@@ -48,6 +67,62 @@ class C
 
             var expected = @"// test
 
+class C
+{
+}";
+            Verify(source, expected);
+
+        }
+
+        [Fact]
+        public void CSharpPreserveExistingMultiline()
+        {
+            _options.CopyrightHeader = ImmutableArray.Create("test1", "test2");
+            var source = @"// test1
+// test2
+
+class C
+{
+}";
+
+            var expected = @"// test1
+// test2
+
+class C
+{
+}";
+            Verify(source, expected);
+
+        }
+
+        [Fact]
+        public void CSharpPreserveExistingWithCommentMultiline()
+        {
+            _options.CopyrightHeader = ImmutableArray.Create("test1", "test2");
+            var source = @"// test1
+// test2
+
+
+
+
+
+// test3
+
+
+class C
+{
+}";
+
+            var expected = @"// test1
+// test2
+
+
+
+
+
+// test3
+
+
 class C
 {
 }";

src/Microsoft.DotNet.CodeFormatting/Rules/CopyrightHeaderRule.cs

@@ -79,6 +79,7 @@ private List<string> GetExistingHeader(SyntaxTriviaList triviaList)
                 {
                     headerList.Add(GetCommentText(triviaList[i].ToFullString()));
                     i++;
+                    MoveToNextLineOrTrivia(triviaList, ref i);
                 }
 
                 return headerList;
@@ -113,10 +114,19 @@ private SyntaxTriviaList RemoveExistingHeader(SyntaxTriviaList oldList)
 
             private void MovePastBlankLines(SyntaxTriviaList list, ref int index)
             {
-                while (index < list.Count && IsWhiteSpaceOrNewLine(list[index]))
+                while (MoveToNextLineOrTrivia(list, ref index))
+                {
+                }
+            }
+            
+            private bool MoveToNextLineOrTrivia(SyntaxTriviaList list, ref int index)
+            {
+                bool moved = index < list.Count && IsWhiteSpaceOrNewLine(list[index]);
+                if (moved)
                 {
                     index++;
                 }
+                return moved;
             }
 
             protected abstract SyntaxTriviaList CreateTriviaList(IEnumerable<SyntaxTrivia> e);