Fix auto roll update.

Auto roll requires an additional "git fetch" now that it uses
a workdir. Before, the checkout used to be updated
externally.

BUG=408523
LOG=n
TEST=script_test.py
R=tandrii@chromium.org
TBR=tandrii@chromium.org

Review URL: https://codereview.chromium.org/695323002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25079 ce2b1a6d-e550-0410-aec6-3dcde31c8c00