Implement a redirector for gitweb-style URLs.

Add a filter to the host index chain to redirect in the presence of
gitweb-style query parameters. Only handles gitweb URLs using the query
parameter flavor; see gitweb(1) for details.

The intent of this filter is to provide compatibility for preexisting
links to old gitweb URLs to be migrated to Gitiles; of course Gitiles
itself does not create such URLs. For this reason, use HTTP 301 (Moved
Permanently) for the redirect rather than 302 Found.

Include simple but not exhaustive tests; the code block in question is
far more concise than exhaustive tests would be.

Change-Id: Ief7022b7cd419fc0380992071ad6e4a4428a6169

gitiles-servlet/src/test/java/com/google/gitiles/FakeHttpServletResponse.java

diff --git a/gitiles-servlet/src/test/java/com/google/gitiles/FakeHttpServletResponse.java b/gitiles-servlet/src/test/java/com/google/gitiles/FakeHttpServletResponse.java
index 5249e41..a6c90e3 100644
--- a/gitiles-servlet/src/test/java/com/google/gitiles/FakeHttpServletResponse.java
+++ b/gitiles-servlet/src/test/java/com/google/gitiles/FakeHttpServletResponse.java
@@ -16,9 +16,11 @@
 
 import static com.google.common.base.Charsets.UTF_8;
 import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Preconditions.checkNotNull;
 import static com.google.common.base.Preconditions.checkState;
 
 import com.google.common.base.Charsets;
+import com.google.common.collect.Iterables;
 import com.google.common.collect.LinkedListMultimap;
 import com.google.common.collect.ListMultimap;
 import com.google.common.net.HttpHeaders;
@@ -252,4 +254,8 @@
   public String getActualBodyString() {
     return RawParseUtils.decode(getActualBody());
   }
+
+  public String getHeader(String name) {
+    return Iterables.getFirst(headers.get(checkNotNull(name)), null);
+  }
 }