Fix branch for resetting change log in push-to-trunk.

Fix branch for resetting change log in push-to-trunk.

The patch is applied to ChangeLog with the "--index" option, which already updates the index of the file. For resetting the ChangeLog to the trunk revision, the trunk branch needs to be stated explicitly in the checkout, otherwise it checks out the already patched HEAD.

TEST=tools/push-to-trunk/script_test.py
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/199993005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@19996 ce2b1a6d-e550-0410-aec6-3dcde31c8c00