Use exactRef() when possible

This avoids ambiguous lookups when refs/heads/refs/meta/config exists.
Instead only refs/meta/config or nothing is returned.

exactRef() can also be optimized by JGit implementations to lookup a
ref, potentially saving time by avoiding unnecessary reads of the
storage system.

Change-Id: I1fd6750e7dce935597cf46331fe7cb6dc21473bb

gerrit-server/src/main/java/com/google/gerrit/server/git/VersionedMetaData.java

@@ -96,7 +96,7 @@ public abstract class VersionedMetaData {
    * @throws ConfigInvalidException
    */
   public void load(Repository db) throws IOException, ConfigInvalidException {
-    Ref ref = db.getRef(getRefName());
+    Ref ref = db.getRefDatabase().exactRef(getRefName());
     load(db, ref != null ? ref.getObjectId() : null);
   }