Allow to push a tag that points to a non-commit object

This was fixed in 2.11.1 with change I013c75db9 but was then
inadvertently reverted by change If4a2375db which was included
in 2.11.2 and 2.11.3.

Redo the original fix.

Bug: Issue 3328
Change-Id: I85e5e1afc65b1408acd0754f6adbdb6742928aaf

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

@@ -2382,7 +2382,11 @@ public class ReceiveCommits {
     walk.sort(RevSort.NONE);
     try {
       Set<ObjectId> existing = Sets.newHashSet();
-      walk.markStart(walk.parseCommit(cmd.getNewId()));
+      RevObject parsedObject = walk.parseAny(cmd.getNewId());
+      if (!(parsedObject instanceof RevCommit)) {
+        return;
+      }
+      walk.markStart((RevCommit)parsedObject);
       markHeadsAsUninteresting(walk, existing, cmd.getRefName());
 
       RevCommit c;