Set path to null for archive link of root tree

Prior to this change, the archive link for the root tree was of the
form "revision/.ext"; this makes it "revision.ext", meaning the
archive of the root tree is identical to (the same URL and therefore
contents as) the archive of the commit.

Change-Id: Ied963a6ecb8c5c8b054c18b1f76e59d980c24938

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

diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/TreeSoyData.java b/gitiles-servlet/src/main/java/com/google/gitiles/TreeSoyData.java
index b7a9b28..0a828c2 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/TreeSoyData.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/TreeSoyData.java
@@ -18,6 +18,7 @@
 
 import com.google.common.annotations.VisibleForTesting;
 import com.google.common.base.Charsets;
+import com.google.common.base.Strings;
 import com.google.common.collect.Lists;
 import com.google.common.collect.Maps;
 import com.google.gitiles.PathServlet.FileType;
@@ -134,6 +135,7 @@
       data.put("logUrl", GitilesView.log().copyFrom(view).toUrl());
       data.put("archiveUrl", GitilesView.archive()
           .copyFrom(view)
+          .setPathPart(Strings.emptyToNull(view.getPathPart()))
           .setExtension(archiveFormat.getDefaultSuffix())
           .toUrl());
       data.put("archiveType", archiveFormat.getShortName());