subrepo: do not try to get hidden revisions

If a subrepo revision is hidden (because it was amended, for example) it does
not make sense to try to "get" it from the remote subrepository.

Note that in order to avoid making the change look bigger than it is, this adds
an unnecessary else clause. This will be removed on a follow up patch.

--HG--
branch : stable
subrepo: remove unnecessary else clause in hgsubrepo._get

This revision has no behaviour change. It simply removes an unnecessary else
that follows an if / return block. The change looks big because a big chunk of
code has been unindented one level.

--HG--
branch : stable
subrepo: make it possible to update to hidden subrepo revisions

When a subrepo revision was hidden it was considered missing and mercurial was
unable to update to the corresponding parent revision. Instead warn the user of
the problem and let it choose what to do (the default is to udpate anyway).

--HG--
branch : stable