Shallow fetch: Pass a DepthWalk to PackWriter

d385a7a5e5 (Shallow fetch: Respect "shallow" lines, 2016-08-03) forgot
that UploadPack wasn't passing a DepthWalk to PackWriter in the first
place.  As a result, shallow clones fail:

  java.lang.IllegalArgumentException: Shallow packs require a DepthWalk
        at org.eclipse.jgit.internal.storage.pack.PackWriter.preparePack(PackWriter.java:756)
        at org.eclipse.jgit.transport.UploadPack.sendPack(UploadPack.java:1497)
        at org.eclipse.jgit.transport.UploadPack.sendPack(UploadPack.java:1381)
        at org.eclipse.jgit.transport.UploadPack.service(UploadPack.java:774)
        at org.eclipse.jgit.transport.UploadPack.upload(UploadPack.java:667)
        at org.eclipse.jgit.http.server.UploadPackServlet.doPost(UploadPackServlet.java:191)

Change-Id: Ib0d8c2946eebfea910a2b767fb92e23da15d4749

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

@@ -1484,16 +1484,19 @@ public class UploadPack {
 				pw.setTagTargets(tagTargets);
 			}
 
-			if (depth > 0)
+			RevWalk rw = walk;
+			if (depth > 0) {
 				pw.setShallowPack(depth, unshallowCommits);
+				rw = new DepthWalk.RevWalk(walk.getObjectReader(), depth);
+				rw.assumeShallow(clientShallowCommits);
+			}
 
-			RevWalk rw = walk;
 			if (wantAll.isEmpty()) {
 				pw.preparePack(pm, wantIds, commonBase);
 			} else {
 				walk.reset();
 
-				ObjectWalk ow = walk.toObjectWalkWithSameObjects();
+				ObjectWalk ow = rw.toObjectWalkWithSameObjects();
 				pw.preparePack(pm, ow, wantAll, commonBase);
 				rw = ow;
 			}