Fix dry_run run() return type to match check_output().

fetch.py

 #!/usr/bin/env python
 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 """
 Tool to perform checkouts in one easy command line!
 
 Usage:
   fetch <recipe> [--property=value [--property2=value2 ...]]
@@ skipping 44 matching lines @@
 
   def init(self):
     pass
 
   def sync(self):
     pass
 
   def run(self, cmd, **kwargs):
     print 'Running: %s' % (' '.join(pipes.quote(x) for x in cmd))
     if self.options.dry_run:
-      return 0
+      return ''
     return subprocess.check_output(cmd, **kwargs)
 
 
 class GclientCheckout(Checkout):
 
   def run_gclient(self, *cmd, **kwargs):
     if not spawn.find_executable('gclient'):
       cmd_prefix = (sys.executable, os.path.join(SCRIPT_PATH, 'gclient.py'))
     else:
       cmd_prefix = ('gclient',)
@@ skipping 260 matching lines @@
   spec, root = run_recipe_fetch(recipe, props)
   return run(options, spec, root)
 
 
 if __name__ == '__main__':
   try:
     sys.exit(main())
   except KeyboardInterrupt:
     sys.stderr.write('interrupted\n')
     sys.exit(1)