optimize revlog.lookup when passed hex(node)[:...]

Instead of converting each node from the filenode to a hex form,
convert the arg to a bin form.
For a revlog with 26711 entries, doing 100 lookup:
before: ~18s
after : ~13s

mercurial/revlog.py

@@ -763,15 +763,22 b' class revlog(object):'
             return self.node(rev)
         except (ValueError, OverflowError):
             pass
+        try:
         # hex(node)[:...]
+            if len(id) % 2 == 0:
+                bin_id = bin(id)
+            else:
+                bin_id = bin(id[:-1])
         node = None
         for n in self.nodemap:
-            if hex(n).startswith(id):
+                if n.startswith(bin_id) and hex(n).startswith(id):
                 if node is not None:
                     raise RevlogError(_("Ambiguous identifier"))
                 node = n
         if node is not None:
             return node
+        except TypeError:
+            pass
 
         # might need fixing if we change hash lengths
         if len(id) == 20 and id in self.nodemap: