convert: fix crazy rollback call, broken by recent rollback safety checks

This was causing test-convert-cvs.t to fail.

hgext/convert/hg.py

@@ -178,7 +178,7 b' class mercurial_sink(converter_sink):'
             closed = 'close' in commit.extra
             if not closed and not man.cmp(m1node, man.revision(mnode)):
                 self.ui.status(_("filtering out empty revision\n"))
-                self.repo.rollback()
+                self.repo.rollback(force=True)
                 return parent
         return p2