Make BaseReceivePack.setAtomic public

This makes atomic more consistent with other properties like
allowNonFastForwards.

Change-Id: Ie3b41f14ffba1d0b206ffecc697377417daa6973

org.eclipse.jgit/src/org/eclipse/jgit/transport/BaseReceivePack.java

 	 *            transaction.
 	 * @since 4.4
 	 */
-	protected void setAtomic(boolean atomic) {
+	public void setAtomic(boolean atomic) {
 		this.atomic = atomic;
 	}