Remove old branch logic from push scripts.

The old temporary branch is not needed anymore. All scripts create a different branch after creating the temporary branch.

This also fixes logging subprocesses and logs a warning to track down script failures due to wrong branches.

BUG=
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/266083003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@21147 ce2b1a6d-e550-0410-aec6-3dcde31c8c00