Add --reset-to-deps and improve comments

gclient_scm.py

 # Copyright (c) 2009 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.
 
 """Gclient-specific SCM-specific operations."""
 
 import logging
 import os
 import posixpath
 import re
@@ skipping 385 matching lines @@
     All reverted files will be appended to file_list.
     """
     __pychecker__ = 'unusednames=args'
     path = os.path.join(self._root_dir, self.relpath)
     if not os.path.isdir(path):
       # revert won't work if the directory doesn't exist. It needs to
       # checkout instead.
       print("\n_____ %s is missing, synching instead" % self.relpath)
       # Don't reuse the args.
       return self.update(options, [], file_list)
-    merge_base = self._Run(['merge-base', 'HEAD', 'origin'])
-    files = self._Run(['diff', merge_base, '--name-only']).split()
-    self._Run(['reset', '--hard', merge_base], redirect_stdout=False)
+
+    default_rev = "refs/heads/master"
+    url, deps_revision = gclient_utils.SplitUrlRevision(self.url)
+    if not deps_revision:
+      deps_revision = default_rev
+    if deps_revision.startswith('refs/heads/'):
+      deps_revision = deps_revision.replace('refs/heads/', 'origin/')
+
+    files = self._Run(['diff', deps_revision, '--name-only']).split()
+    self._Run(['reset', '--hard', deps_revision], redirect_stdout=False)
     file_list.extend([os.path.join(self.checkout_path, f) for f in files])
 
   def revinfo(self, options, args, file_list):
     """Display revision"""
     __pychecker__ = 'unusednames=options,args,file_list'
     return self._Run(['rev-parse', 'HEAD'])
 
   def runhooks(self, options, args, file_list):
     self.status(options, args, file_list)
 
@@ skipping 433 matching lines @@
       print("\n________ couldn't run \'%s\' in \'%s\':\nThe directory "
             "does not exist."
             % (' '.join(command), path))
       # There's no file list to retrieve.
     else:
       scm.SVN.RunAndGetFileList(options, command, path, file_list)
 
   def FullUrlForRelativeUrl(self, url):
     # Find the forth '/' and strip from there. A bit hackish.
     return '/'.join(self.url.split('/')[:4]) + url