Rename new sortkey index field to sortkey2

For technical reasons googlesource.com's secondary index
implementation needs to construct a document containing the union of
all fields from all active write schema versions; for this reason we
don't allow distinct fields in different versions to have the same
name.  This fact was missed in Icb760dba.

Don't bump the schema version since that change has only been
submitted for a few hours.

Change-Id: I4f8a2e515372be3a4b4a145a0b54eef864dcea00

gerrit-server/src/test/java/com/google/gerrit/server/index/IndexRewriteTest.java

@@ -54,7 +54,7 @@ public class IndexRewriteTest extends TestCase {
     rewrite = new IndexRewriteImpl(
         indexes,
         null,
-        new IndexRewriteImpl.BasicRewritesImpl(null),
+        new IndexRewriteImpl.BasicRewritesImpl(null, indexes),
         new SqlRewriterImpl(null));
   }