Make release script cleanup more robust.

This is a work-around that checks out master if the
remembered branch is corrupt.

BUG=chromium:410721
LOG=n
TEST=script_test.py
TBR=tandrii@chromium.org

Review URL: https://codereview.chromium.org/659473002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24599 ce2b1a6d-e550-0410-aec6-3dcde31c8c00