[MERGE][#365865] Implement RemoteBranch.set_append_revisions_only.

[Robert Collins]

bb:tweak

Lets pull this up to the base Branch rather than duplicating code, and
move the check for 'do I support this' into the branch config - that
should be pretty straight forward and won't require _ensure_real, but
will let the server DTRT on old branches.

-Rob
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