Explicitly use "tip" in revision navigation.

Rationale: the tip revision is a moving target and if somebody wants tip,
it should be delivered.

mercurial/hgweb/hgweb_mod.py

@@ -60,7 +60,7 b' def revnavgen(pos, pagelen, limit, nodef'
             for label, node in l:
                 yield {"label": label, "node": node}
 
-            yield {"label": "tip", "node": hex(nodefunc('-1').node())}
+            yield {"label": "tip", "node": "tip"}
         except hg.RepoError:
             pass