This warning exists to prevent git users from prematurely polluting
their namespace when trying out Mercurial. But for repos that already
have multiple branches, understanding what branches are is not
optional so we should just shut up.
When issuing `hg pull -r REV` in a repo with no common ancestor with the
remote repo, the message 'requesting all changes' is printed, even though only
the changese that are ancestors of REV are actually requested. This can be
confusing for users (see
http://www.selenic.com/pipermail/mercurial/2010-October/035508.html).
This silences the message if (and only if) the '-r' option was passed.
