Add tag write access to merge script.

The script will poll the remote branch until the git updater
has replicated the commit to tag. The tag will then be
directly pushed to git.

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

Review URL: https://codereview.chromium.org/607893004

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24431 ce2b1a6d-e550-0410-aec6-3dcde31c8c00