Get Config for a request from the GitilesAccess

Allow for per-repo configuration by reading config values on each
request rather than in servlet constructors. Reading a single config
value is cheap enough, but parsing configs from the on-disk format is
potentially expensive; leave this optimization up to the
GitilesAccess implementation.

Currently, in DefaultAccess, we just pass in the global config, no
per-repo config. This is simple expediency because Config objects
don't trivially support merging multiple configs except by
"inheritance".

Because of the way we were caching per-config maps of extension to
ArchiveFormat, this requires some more substantial refactoring to
that class and ArchiveServlet.

Change-Id: Ia1fcf97ecc71571537402629f49e8a65128a105e

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

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/GitilesAccess.java b/gitiles-servlet/src/main/java/com/google/gitiles/GitilesAccess.java
index 648d709..978baa3 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/GitilesAccess.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/GitilesAccess.java
@@ -14,6 +14,7 @@
 
 package com.google.gitiles;
 
+import org.eclipse.jgit.lib.Config;
 import org.eclipse.jgit.transport.resolver.ServiceNotAuthorizedException;
 import org.eclipse.jgit.transport.resolver.ServiceNotEnabledException;
 
@@ -65,4 +66,10 @@
    *         the repository.
    */
   public RepositoryDescription getRepositoryDescription() throws IOException;
+
+  /**
+   * @return configuration to apply to the host/repository for this request.
+   * @throws IOException an error occurred reading the configuration.
+   */
+  public Config getConfig() throws IOException;
 }