getgraphlogrevs: return an empty baseset instead of a empty list

We aims at returning smartset only so that function higher in the stack can use
smartset feature.

mercurial/cmdutil.py

@@ -1649,7 +1649,7 b' def getgraphlogrevs(repo, pats, opts):'
             revs = revset.spanset(repo)
             revs.reverse()
     if not revs:
-        return [], None, None
+        return revset.baseset(), None, None
     expr, filematcher = _makegraphlogrevset(repo, pats, opts, revs)
     if possiblyunsorted:
         revs.sort(reverse=True)