pullrequests: better handling of Mercurial pullrequests with missing revisions - don't crash

Trying to display a Mercurial PR with missing changesets could give a crash
when trying to compute available updates after 3f646f7bac39 did that
c.cs_ranges could be empty. That would normally not happen on Mercurial, but
could happen when restoring an old filesystem backup ... or when using strip on
the server.