Fix IncorrectObjectTypeException on initial commit

On the initial commit of a project, "a" is the empty tree,
not a commit object, in order to show all initial files as
new files.  To cast it into a RevTree we must use parseTree()
and not parseCommit().getTree().  For all subsequent changes, "a"
will be a commit and parseTree() will automatically deference to
the commit's root tree.

Bug: issue 442
Change-Id: I9302b6e3d6e9117159e32e96119b6563ac8e26c1
Signed-off-by: Shawn O. Pearce <sop@google.com>

gerrit-server/src/main/java/com/google/gerrit/server/patch/PatchListCacheImpl.java

@@ -174,7 +174,7 @@ public class PatchListCacheImpl implements PatchListCache {
       }
     }
 
-    RevTree aTree = a != null ? rw.parseCommit(a).getTree() : null;
+    RevTree aTree = a != null ? rw.parseTree(a) : null;
     RevTree bTree = b.getTree();
 
     final int cnt = p.getFiles().size();