When using `hg push --rev X`, the subset considered by discovery is only `::X`.
In addition, `X` can be any local revisions not just local heads. As a result
the message "all local heads known locally" can be misleading. We replace it
with the more accurate "all local changesets known remotely".

The message appears when in verbose not, so this is stricly speaking a BC
breakage. I am not sure this would be a real issue in practice...