return found path directly in which

Library/Homebrew/utils.rb

@@ -170,11 +170,11 @@ def puts_columns items, star_items=[]
 end
 
 def which cmd, path=ENV['PATH']
-  dir = path.split(File::PATH_SEPARATOR).find {|p|
+  path.split(File::PATH_SEPARATOR).find do |p|
     pcmd = File.join(p, cmd)
-    File.executable?(pcmd) && !File.directory?(pcmd)
-  }
-  Pathname.new(File.join(dir, cmd)) unless dir.nil?
+    return pcmd if File.executable?(pcmd) && !File.directory?(pcmd)
+  end
+  return nil
 end
 
 def which_editor