Show "∞" when there's a circular alias reference.

test/fast/Aliases/setup

@@ -0,0 +1,9 @@
+#!/bin/sh
+
+for i in $(seq 1 10)
+  do
+  echo 0.0.$i > ../../../alias/test-stable-$i
+  mkdir -p ../../../v0.0.$i
+  echo 0.1.$i > ../../../alias/test-unstable-$i
+  mkdir -p ../../../v0.1.$i
+done