rebaseline_server: add "prefetch" directive that just warms the cache without awaiting results

gm/rebaseline_server/imagepair_test.py

 #!/usr/bin/python
 
 """
 Copyright 2014 Google Inc.
 
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 
 Test imagepair.py
 """
@@ skipping 143 matching lines @@
                 },
                 'extraColumns': {
                     'builder': 'MyBuilder',
                     'test': 'MyTest',
                 },
                 'imageAUrl': 'arcofzorro/16206093933823793653.png',
                 'imageBUrl': 'nonexistentDir/111111.png',
                 'isDifferent': True,
             },
         ],
+
+        # One of the two images is missing, but download_all_images=True so we
+        # should download it anyway.
+        [
+            # inputs:
+            None,
+            'arcofzorro/13786535001616823825.png',
+            None,
+            None,
+            # expected output:
+            {
+                'imageAUrl': None,
+                'imageBUrl': 'arcofzorro/13786535001616823825.png',
+                'isDifferent': True,
+            },
+        ],
+
     ]
 
     db = imagediffdb.ImageDiffDB(self.temp_dir)
     for selftest in selftests:
       image_pair = imagepair.ImagePair(
           image_diff_db=db,
           base_url=IMG_URL_BASE,
           imageA_relative_url=selftest[0],
           imageB_relative_url=selftest[1],
           expectations=selftest[2],
-          extra_columns=selftest[3])
+          extra_columns=selftest[3],
+          download_all_images=True)
       self.assertEqual(image_pair.as_dict(), selftest[4])
 
 
 def main():
   suite = unittest.TestLoader().loadTestsFromTestCase(ImagePairTest)
   unittest.TextTestRunner(verbosity=2).run(suite)
 
 
 if __name__ == '__main__':
   main()