UploadPack: restore inadvertently deleted line

In 7b96bd812e ("UploadPack: Use reachability checker to validate
non-advertised wants", 2019-05-16), a "walk.setRetainBody(false);"
statement was inadvertently deleted. (An earlier version of this commit
had this line in another part of the code and a review comment suggested
to move it back here; the line was then deleted from the other part of
the code but not readded.) Restore this line.

Change-Id: I96ff6106ba9e4eef429388c83e898b3363295f69
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>

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

@@ -1876,6 +1876,7 @@ public class UploadPack {
 
 		ObjectReader reader = up.getRevWalk().getObjectReader();
 		try (RevWalk walk = new RevWalk(reader)) {
+			walk.setRetainBody(false);
 			// Missing "wants" throw exception here
 			List<RevObject> wantsAsObjs = objectIdsToRevObjects(walk,
 					notAdvertisedWants);