Fix merging changes into an empty repository

If an initial change for an empty repository is pushed and the submit
type is MERGE_ALWAYS, on submit the change gets stuck in the
'Submitted, Merge Pending' state. Merging the change fails because
the repository is empty and there is no commit with which the merge
can be done. When trying to do the merge there is a
NullPointerException because mergeTip is null.

The same NullPointerException can happen with the MERGE_IF_NECESSARY
submit type. Here the initial change can be successfully merged
because this will be done as a fast forward. However if in this
situation there would be other changes to be merged in the queue and a
merge would be necessary for one of them the NullPointerException
would occur.

Change-Id: Id1526eb8759dbac028a5b2c2a76b1dbea7c330a6
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>

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

@@ -25,9 +25,16 @@ public class MergeIfNecessary extends SubmitStrategy {
   }
 
   @Override
-  protected CodeReviewCommit _run(final CodeReviewCommit mergeTip,
-      final List<CodeReviewCommit> toMerge) throws MergeException {
+  protected CodeReviewCommit _run(CodeReviewCommit mergeTip,
+      List<CodeReviewCommit> toMerge) throws MergeException {
     args.mergeUtil.reduceToMinimalMerge(args.mergeSorter, toMerge);
+
+    if (mergeTip == null) {
+      // The branch is unborn. Take a fast-forward resolution to
+      // create the branch.
+      mergeTip = toMerge.remove(0);
+    }
+
     CodeReviewCommit newMergeTip =
         args.mergeUtil.getFirstFastForward(mergeTip, args.rw, toMerge);