largefiles: use filechunkiter for iterating largefile when serving getlfile

The default file iterator is line based and will give odd chunk sizes - often
very short and relatively expensive.

hgext/largefiles/proto.py

@@ -63,7 +63,7 b' def getlfile(repo, proto, sha):'
     # ssh proto does for string responses.
     def generator():
         yield '%d\n' % length
-        for chunk in f:
+        for chunk in util.filechunkiter(f):
             yield chunk
     return wireproto.streamres(generator())