Also simplify tagging in merge_to_branch.

BUG=chromium:410721
LOG=n
R=jkummerow@chromium.org, tandrii@chromium.org

Review URL: https://codereview.chromium.org/607463002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@24228 ce2b1a6d-e550-0410-aec6-3dcde31c8c00