Fix markdown docstring wrapping (palantir#1672)

Fix issue when wrapping markdown docstrings resulted in 2 slashes instead of 3

Author: DHerls

palantir-java-format/src/main/java/com/palantir/javaformat/java/JavaCommentsHelper.java

@@ -113,6 +113,14 @@ private String indentLineComments(List<String> lines, int column0) {
     private static final Pattern LINE_COMMENT_MISSING_SPACE_PREFIX =
             Pattern.compile("^(//+)(?!noinspection|\\$NON-NLS-\\d+\\$)[^\\s/]");
 
+    private static String lineCommentPrefix(String line) {
+        int prefixLength = 0;
+        while (prefixLength < line.length() && line.charAt(prefixLength) == '/') {
+            prefixLength++;
+        }
+        return "/".repeat(prefixLength);
+    }
+
     private List<String> wrapLineComments(List<String> lines, int column0) {
         List<String> result = new ArrayList<>();
         for (String line : lines) {
@@ -127,17 +135,19 @@ private List<String> wrapLineComments(List<String> lines, int column0) {
                 result.add(line);
                 continue;
             }
+            // Preserve the original slash prefix (e.g. `///` for markdown docstrings) on wrapped lines.
+            String prefix = lineCommentPrefix(line);
             while (line.length() + column0 > options.maxLineLength()) {
                 int idx = options.maxLineLength() - column0;
                 // only break on whitespace characters, and ignore the leading `// `
-                while (idx >= 2 && !CharMatcher.whitespace().matches(line.charAt(idx))) {
+                while (idx >= prefix.length() && !CharMatcher.whitespace().matches(line.charAt(idx))) {
                     idx--;
                 }
-                if (idx <= 2) {
+                if (idx <= prefix.length()) {
                     break;
                 }
                 result.add(line.substring(0, idx));
-                line = "//" + line.substring(idx);
+                line = prefix + line.substring(idx);
             }
             result.add(line);
         }

palantir-java-format/src/test/java/com/palantir/javaformat/java/FormatterTest.java

@@ -430,6 +430,22 @@ public void blankLinesImportComment() throws FormatterException {
         assertThat(ImportOrderer.reorderImports(withoutBlank, Style.GOOGLE)).isEqualTo(withoutBlank);
     }
 
+    @Test
+    public void wrapMarkdownDocstringComment() throws Exception {
+        String input = "class T {\n"
+                + "  /// one long incredibly unbroken sentence moving from topic to topic so that no-one"
+                + " had a chance to interrupt;\n"
+                + "  void m() {}\n"
+                + "}\n";
+        String expected = "class T {\n"
+                + "  /// one long incredibly unbroken sentence moving from topic to topic so that no-one"
+                + " had a chance\n"
+                + "  /// to interrupt;\n"
+                + "  void m() {}\n"
+                + "}\n";
+        assertThat(Formatter.create().formatSource(input)).isEqualTo(expected);
+    }
+
     @Test
     public void dontWrapMoeLineComments() throws Exception {
         assertThat(Formatter.create()