Do not enforce DeltaWindow maxMemory when zero.

The maxMemory for a DeltaWindow can be optionally disabled when it is
less than or equal to zero. Respect this configuration when enforcing
the limits on object load.

Change-Id: Ic0f4ffcabf82105f8e690bd0eb5e6be485a313b3

org.eclipse.jgit/src/org/eclipse/jgit/storage/pack/DeltaWindow.java

@@ -460,6 +460,9 @@ class DeltaWindow {
 	}
 
 	private void checkLoadable(DeltaWindowEntry ent, long need) {
+		if (maxMemory <= 0)
+			return;
+
 		int tail = next(resSlot);
 		while (maxMemory < loaded + need) {
 			DeltaWindowEntry cur = window[tail];