pw.setReuseDeltaCommits(false);
if (tagTargets != null)
pw.setTagTargets(tagTargets);
if (excludeObjects != null)
for (ObjectIdSet idx : excludeObjects)
pw.excludeObjects(idx);
pw.preparePack(pm, want, have);
if (pw.getObjectCount() == 0)
return null;
// create temporary files