lfs: explicitly name a key when sorting blob pointers

This is needed to keep py3 happy.  The other two instances of sorting already
did this.

hgext/lfs/wrapper.py

@@ -333,7 +333,7 b' def extractpointers(repo, revs):'
             for p in pointersfromctx(ctx).values():
                 pointers[p.oid()] = p
             progress.increment()
-        return sorted(pointers.values())
+        return sorted(pointers.values(), key=lambda p: p.oid())
 
 def pointerfromctx(ctx, f, removed=False):
     """return a pointer for the named file from the given changectx, or None if

tests/test-lfs.t

@@ -876,8 +876,6 b' avoids the corrupt lfs object in the ori'
   pushing to dest
   searching for changes
   lfs: found 22f66a3fc0b9bf3f012c814303995ec07099b3a9ce02a7af84b5970811074a3b in the local lfs store
-  lfs: found 89b6070915a3d573ff3599d1cda305bc5e38549b15c4847ab034169da66e1ca8 in the local lfs store
-  lfs: found b1a6ea88da0017a0e77db139a54618986e9a2489bee24af9fe596de9daac498c in the local lfs store
   abort: detected corrupt lfs object: 66100b384bf761271b407d79fc30cdd0554f3b2c5d944836e936d584b88ce88e
   (run hg verify)
   [255]