When applying a gerrit patch, do a checkout before soft resetting.

scripts/slave/bot_update.py

 #!/usr/bin/env python
 # Copyright 2014 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.
 
 # TODO(hinoka): Use logging.
 
 import cStringIO
 import codecs
 import collections
@@ skipping 1089 matching lines @@
   # Only try once, since subsequent failures hide the real failure.
   try:
     call(*cmd, tries=1)
   except SubprocessFailed as e:
     raise PatchFailed(e.message, e.code, e.output)
 
 def apply_gerrit_ref(gerrit_repo, gerrit_ref, root):
   gerrit_repo = gerrit_repo or 'origin'
   assert gerrit_ref
   try:
+    base_rev = git('rev-parse', 'HEAD', cwd=root).strip()
     git('retry', 'fetch', gerrit_repo, gerrit_ref, cwd=root, tries=1)
-    git('reset', '--soft', 'FETCH_HEAD', cwd=root)
+    git('checkout', 'FETCH_HEAD', cwd=root)
+    git('reset', '--soft', base_rev, cwd=root)
   except SubprocessFailed as e:
     raise PatchFailed(e.message, e.code, e.output)
 
 def check_flag(flag_file):
   """Returns True if the flag file is present."""
   return os.path.isfile(flag_file)
 
 
 def delete_flag(flag_file):
   """Remove bot update flag."""
@@ skipping 578 matching lines @@
   except Exception:
     # Unexpected failure.
     emit_flag(options.flag_file)
     raise
   else:
     emit_flag(options.flag_file)
 
 
 if __name__ == '__main__':
   sys.exit(main())