Paper bag fix repo 1.3's "repo upload" without --replace

If we aren't doing a replacement we do not have any
replace rules, so we cannot iterate over them.

Signed-off-by: Shawn O. Pearce <sop@google.com>

gerrit_upload.py

@@ -114,6 +114,7 @@ def UploadBundle(project,
         req.dest_branch = str(dest_branch)
         for c in revlist:
           req.contained_object.append(c)
+        if replace_changes:
           for change_id,commit_id in replace_changes.iteritems():
             r = req.replace.add()
             r.change_id = change_id