Ensure the ObjectInserter flushes after a merge

If this does not happen some databases may discard
objects and not make them available.

Change-Id: I347b3c3724db52c8a6c09f4804071497a3a377ab

org.eclipse.jgit/src/org/eclipse/jgit/merge/Merger.java

@@ -153,7 +153,10 @@ public abstract class Merger {
 			sourceTrees[i] = walk.parseTree(sourceObjects[i]);
 
 		try {
-			return mergeImpl();
+			boolean ok = mergeImpl();
+			if (ok && inserter != null)
+				inserter.flush();
+			return ok;
 		} finally {
 			if (inserter != null)
 				inserter.release();