Markdown: render pages under /+doc/

Parse and render any file ending with ".md" as markdown using the
pegdown processor.  This allows a repository to be used as a
documentation source.

If "navbar.md" is found in the top level directory it is rendered as a
horizontal bar across the top of the page.  A navbar.md is an outline:

  * [Home](/index.md)
  * [APIs](/api/index.md)
  * [Source](/src/main/java/index.md)

Any links starting with "/" and ending in ".md" are resolved as
relative to the top of the Git repository.

As an extension to Markdown the special block [TOC] is replaced with
a table of contents from the headers of the text.

Markdown support can be optionally disabled by setting the config
variable markdown.render to false.

Change-Id: Ic111628c59cfadfdca37bf0cc637ee8a14d54c37

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

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/doc/MarkdownHelper.java b/gitiles-servlet/src/main/java/com/google/gitiles/doc/MarkdownHelper.java
index 6d67505..e7566f5 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/doc/MarkdownHelper.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/doc/MarkdownHelper.java
@@ -16,6 +16,7 @@
 
 import com.google.common.base.Strings;
 
+import org.pegdown.ast.HeaderNode;
 import org.pegdown.ast.Node;
 import org.pegdown.ast.TextNode;
 
@@ -48,6 +49,23 @@
     }
   }
 
+  static String getTitle(Node node) {
+    if (node instanceof HeaderNode) {
+      if (((HeaderNode) node).getLevel() == 1) {
+        return getInnerText(node);
+      }
+      return null;
+    }
+
+    for (Node child : node.getChildren()) {
+      String title = getTitle(child);
+      if (title != null) {
+        return title;
+      }
+    }
+    return null;
+  }
+
   private MarkdownHelper() {
   }
 }