Refactor MarkdownToHtml with Builder pattern

This refactoring simplifies how we access information about the
document being rendered, which makes the navbar.md, README.md and
normal markdown files all more consistent.

MarkdownConfig is now a JGit SectionParser managed object, which
allows caching the immutable MarkdownConfig inside the JGit Config.
This consolidates the key references from all over the code to one
location, and may improve performance for servers that reuse the
Config object across requests.

Change-Id: Id85130d1bc7351f125bf3b0f8bf3c6e028272b30

gitiles-servlet/src/main/java/com/google/gitiles/RepositoryIndexServlet.java

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/RepositoryIndexServlet.java b/gitiles-servlet/src/main/java/com/google/gitiles/RepositoryIndexServlet.java
index f4d9327..a6758a9 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/RepositoryIndexServlet.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/RepositoryIndexServlet.java
@@ -23,6 +23,7 @@
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.Maps;
 import com.google.gitiles.DateFormatter.Format;
+import com.google.gitiles.doc.MarkdownConfig;
 import com.google.gson.reflect.TypeToken;
 import com.google.template.soy.data.SanitizedContent;
 
@@ -168,7 +169,7 @@
         new ReadmeHelper(
             walk.getObjectReader(),
             GitilesView.path().copyFrom(view).setRevision(Revision.HEAD).setPathPart("/").build(),
-            cfg,
+            MarkdownConfig.get(cfg),
             rootTree);
     readme.scanTree(rootTree);
     if (readme.isPresent()) {