bisect-kit: add snapshot support for ChromeOS prebuilt

Added a `--experimental_use_snapshot` flag to trigger using
prebuilt snapshot image for bisect-kit prebuilt bisection.

BUG=b:140721293
TEST=unittest, ./diagnose_cros_autotest.py and ./diagnose_cros_tast.py with
--experimental_use_snapshot flag

Change-Id: I006f47b1aeb5b0306441f09222646d0f2d2d4ad6
Reviewed-on: https://chromium-review.googlesource.com/1795945
Tested-by: Zheng-Jie Chang <zjchang@chromium.org>
Commit-Ready: Zheng-Jie Chang <zjchang@chromium.org>
Commit-Ready: Chung-yih Wang <cywang@google.com>
Legacy-Commit-Queue: Commit Bot <commit-bot@chromium.org>
Reviewed-by: Kuang-che Wu <kcwu@chromium.org>
Reviewed-by: Chung-yih Wang <cywang@google.com>
diff --git a/bisect_kit/codechange.py b/bisect_kit/codechange.py
index 7649316..7c9701e 100644
--- a/bisect_kit/codechange.py
+++ b/bisect_kit/codechange.py
@@ -840,7 +840,7 @@
     Returns:
       list of rev string
     """
-    logger.info('build_revlist')
+    logger.info('build_revlist: old = %s, new = %s', old, new)
     revlist = []
 
     # step 1, find all float and fixed specs in the given range.
@@ -931,8 +931,8 @@
     cache_dir = os.path.join(self.root_dir, _DIFF_CACHE_DIR)
     cache_filename = os.path.join(cache_dir, '%s,%s.json' % (old, new))
     if not os.path.exists(cache_filename):
-      raise errors.InternalError(
-          'cached revlist not found: %s' % cache_filename)
+      raise errors.InternalError('cached revlist not found: %s' %
+                                 cache_filename)
 
     result = []
     for data in json.load(open(cache_filename)):