A node may not depend on itself

Also simplify generation of exception message. The *print-length*
binding was paranoid: nodes should probably not be large data
structures.

Originally reported by "tgoossens" here:
#1

Also added to tools.namespace:
clojure/tools.namespace@41cd3c3

src/com/stuartsierra/dependency.clj

@@ -71,11 +71,9 @@
                (set (keys dependents))))
   DependencyGraphUpdate
   (depend [graph node dep]
-    (when (depends? graph dep node)
-      (let [^String msg (binding [*print-length* 10]
-                          (str "Circular dependency between "
-                               (pr-str node) " and " (pr-str dep)))]
-        (throw (Exception. msg))))
+    (when (or (= node dep) (depends? graph dep node))
+      (throw (Exception. (str "Circular dependency between "
+                              (pr-str node) " and " (pr-str dep)))))
     (MapDependencyGraph.
      (update-in dependencies [node] set-conj dep)
      (update-in dependents [dep] set-conj node)))