Include GitHub markdown compatibility anchor only as needed

Limit adding the second anchor to provide better compatibility with
GitHub's markdown rendering to be only added if the existing id is not
already all lowercase and thus compatible.

Change-Id: If018d4452263226ef4215f50dd517a3e84a79de6

gitiles-servlet/src/main/java/com/google/gitiles/doc/MarkdownToHtml.java

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/doc/MarkdownToHtml.java b/gitiles-servlet/src/main/java/com/google/gitiles/doc/MarkdownToHtml.java
index 9de1a09..b88f4fd 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/doc/MarkdownToHtml.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/doc/MarkdownToHtml.java
@@ -229,13 +229,15 @@
           .close("span")
           .close("a");
       // github markdown compatibility
-      html.open("a")
-          .attribute("class", "h")
-          .attribute("name", id.toLowerCase())
-          .attribute("href", "#" + id.toLowerCase())
-          .open("span")
-          .close("span")
-          .close("a");
+      if (id != id.toLowerCase()) {
+        html.open("a")
+            .attribute("class", "h")
+            .attribute("name", id.toLowerCase())
+            .attribute("href", "#" + id.toLowerCase())
+            .open("span")
+            .close("span")
+            .close("a");
+      }
     }
     visitChildren(node);
     html.close(tag);

gitiles-servlet/src/test/java/com/google/gitiles/doc/DocServletTest.java

diff --git a/gitiles-servlet/src/test/java/com/google/gitiles/doc/DocServletTest.java b/gitiles-servlet/src/test/java/com/google/gitiles/doc/DocServletTest.java
index 38bbbf9..c27f0ba 100644
--- a/gitiles-servlet/src/test/java/com/google/gitiles/doc/DocServletTest.java
+++ b/gitiles-servlet/src/test/java/com/google/gitiles/doc/DocServletTest.java
@@ -54,11 +54,7 @@
     assertThat(html).contains("<li><a href=\"/b/repo/+/master/index.md\">Home</a></li>");
     assertThat(html).contains("<li><a href=\"/b/repo/+/master/README.md\">README</a></li>");
     assertThat(html)
-        .contains(
-            "<h1>"
-                + "<a class=\"h\" name=\"page\" href=\"#page\"><span></span></a>"
-                + "<a class=\"h\" name=\"page\" href=\"#page\"><span></span></a>"
-                + "page</h1>");
+        .contains("<h1><a class=\"h\" name=\"page\" href=\"#page\"><span></span></a>page</h1>");
   }
 
   @Test
@@ -88,7 +84,6 @@
         .contains(
             "<h1>"
                 + "<a class=\"h\" name=\"debug\" href=\"#debug\"><span></span></a>"
-                + "<a class=\"h\" name=\"debug\" href=\"#debug\"><span></span></a>"
                 + "Section</h1>");
     assertThat(html)
         .contains(