walk.setRevFilter(RevFilter.MERGE_BASE);
walk.markStart(sourceCommits[aIdx]);
walk.markStart(sourceCommits[bIdx]);
final RevCommit base = walk.next();
if (base == null)
return new EmptyTreeIterator();
final RevCommit base2 = walk.next();
if (base2 != null) {
throw new IOException(MessageFormat.format(JGitText.get().multipleMergeBasesFor
, sourceCommits[aIdx].name(), sourceCommits[bIdx].name()
, base.name(), base2.name()));