gclient: handle case for upstream has rebased

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

gclient_scm.py

@@ -135,13 +135,16 @@ class GitWrapper(SCMWrapper, scm.GIT):
       file_list.extend([os.path.join(self.checkout_path, f) for f in files])
       return
 
-    self._Run(['remote', 'update'], redirect_stdout=False)
     new_base = 'origin'
     if revision:
       new_base = revision
+    cur_branch = self._Run(['symbolic-ref', 'HEAD']).split('/')[-1]
+    merge_base = self._Run(['merge-base', 'HEAD', new_base])
+    self._Run(['remote', 'update'], redirect_stdout=False)
     files = self._Run(['diff', new_base, '--name-only']).split()
     file_list.extend([os.path.join(self.checkout_path, f) for f in files])
-    self._Run(['rebase', '-v', new_base], redirect_stdout=False)
+    self._Run(['rebase', '-v', '--onto', new_base, merge_base, cur_branch],
+                redirect_stdout=False)
     print "Checked out revision %s." % self.revinfo(options, (), None)
 
   def revert(self, options, args, file_list):