Repository.writeMergeHeads to accept a list with subtypes of ObjectId

Allows to pass for example a List<RevCommit>.

Change-Id: I5522707060d7d3e0d848ba579956dc62f8fa0234
Signed-off-by: Konrad Kügler <swamblumat-eclipsebugs@yahoo.de>

org.eclipse.jgit/src/org/eclipse/jgit/lib/Repository.java

 	 *            $GIT_DIR/MERGE_HEAD or <code>null</code> to delete the file
 	 * @throws IOException
 	 */
-	public void writeMergeHeads(List<ObjectId> heads) throws IOException {
+	public void writeMergeHeads(List<? extends ObjectId> heads) throws IOException {
 		writeHeadsFile(heads, Constants.MERGE_HEAD);
 	}
 
 	 * @throws FileNotFoundException
 	 * @throws IOException
 	 */
-	private void writeHeadsFile(List<ObjectId> heads, String filename)
+	private void writeHeadsFile(List<? extends ObjectId> heads, String filename)
 			throws FileNotFoundException, IOException {
 		File headsFile = new File(getDirectory(), filename);
 		if (heads != null) {