largefiles: introduce push --lfrev to control which revisions are pushed
The default of pushing all largefiles referenced in outgoing revisions is safe,
but also expensive and sometimes not what is needed. We thus introduce a
--lfrev option, similar to what pull already has.
By specifying an empty set of revisions (or null), it is possible to get lazy
(and insecure!) pushes of revisions without referenced largefiles, similar to
how pull works.