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).
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.