Docserver: Use GitilesFileSystem instead of SubversionFileSystem

chrome/common/extensions/docs/server2/gitiles_file_system.py

 # 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.
 
 from base64 import b64decode
 from itertools import izip
 import json
 import posixpath
 import traceback
 
@@ skipping 157 matching lines @@
     # commit info JSON content.
     fetch_future = self._fetcher.FetchAsync(self._base_url + _JSON_FORMAT)
     content_future = self._ResolveFetchContent(self._base_url, fetch_future)
     return content_future.Then(lambda json: _ParseGitilesJson(json)[key])
 
   def GetCommitID(self):
     '''Returns a future that resolves to the commit ID for this branch.
     '''
     return self._GetCommitInfo('commit')
 
+  def GetPreviousCommitID(self):
+    '''Returns a future that resolves to the previous commit ID for this branch.
+    '''
+    return self._GetCommitInfo('parents').Then(lambda parents: parents[0])
+
   def StatAsync(self, path):
     dir_, filename = posixpath.split(path)
     def stat(content):
       stat_info = _CreateStatInfo(content)
       if stat_info.version is None:
         raise FileSystemError('Failed to find version of dir %s' % dir_)
       if IsDirectory(path):
         return stat_info
       if filename not in stat_info.child_versions:
         raise FileNotFoundError(
             '%s from %s was not in child versions for Stat' % (filename, path))
       return StatInfo(stat_info.child_versions[filename])
     fetch_future = self._FetchAsync(ToDirectory(dir_) + _JSON_FORMAT)
     return self._ResolveFetchContent(path, fetch_future).Then(stat)
 
   def GetIdentity(self):
     # NOTE: Do not use commit information to create the string identity.
     # Doing so will mess up caching.
     if self._commit is None and self._branch != 'master':
       str_id = GITILES_BRANCH_BASE
     else:
       str_id = GITILES_BASE
     return '@'.join((self.__class__.__name__, StringIdentity(str_id)))