gclient sync will potentially fail if the upstream git tree has been rebased.
This CL fixes this by using rebase --onto to only rebase our local changes.

Review URL: http://codereview.chromium.org/432017

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@33011 0039d316-1c4b-4281-b951-d872f2087c98