Docserver: Use GitilesFileSystem instead of SubversionFileSystem

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

Index: chrome/common/extensions/docs/server2/gitiles_file_system.py
diff --git a/chrome/common/extensions/docs/server2/gitiles_file_system.py b/chrome/common/extensions/docs/server2/gitiles_file_system.py
index df907e6b08ab83d9783629f476654cda8230aa7e..e47c1b94a3a94372dd4284288f0cda403b8df008 100644
--- a/chrome/common/extensions/docs/server2/gitiles_file_system.py
+++ b/chrome/common/extensions/docs/server2/gitiles_file_system.py
@@ -175,6 +175,11 @@ class GitilesFileSystem(FileSystem):
     '''
     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):