Refactor ObjectToPack's delta depth setting

Long ago when PackWriter is first written we thought that the delta
depth could be updated automatically.  But its never used.  Instead
make this a simple standard setter so the caller can more directly
set the delta depth of this object.  This permits us to configure a
depth that takes into account more than just the depth of another
object in this same pack.

Change-Id: I1d71b74f2edd7029b8743a2c13b591098ce8cc8f
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

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

@@ -179,14 +179,7 @@ public class ObjectToPack extends PackedObjectInfo {
 		return flags >>> DELTA_SHIFT;
 	}
 
-	void updateDeltaDepth() {
-		final int d;
-		if (deltaBase instanceof ObjectToPack)
-			d = ((ObjectToPack) deltaBase).getDeltaDepth() + 1;
-		else if (deltaBase != null)
-			d = 1;
-		else
-			d = 0;
+	void setDeltaDepth(int d) {
 		flags = (d << DELTA_SHIFT) | (flags & NON_DELTA_MASK);
 	}