[merge][0.91] deprecate Branch.append_revision

[Martin Pool]

I think this is not actually mergeable as is, because different
branches have different behaviour if you try to point them to a
non-existant revision id.  I'll incorporate it in a later patch; it's
just split out for review.

Another option would be to just delete append_revision now since
plugin code is unlikely to use it and the contract is somewhat
unclear.

-- 
Martin