Fix the call to "git checkout" on Windows.
Fix the call to "git checkout" on Windows.

The "^0" is not needed, and it confuses Windows since
the carret is a special character.
Review URL: http://codereview.chromium.org/6976050

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@87636 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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