Teach releases script to read bleeding_edge tags.

The command 'git svn log' does unfortunately not provide the tag-revision relation. We therefore use 'svn log' to retrieve it from the remote repository.

BUG=
R=jarin@chromium.org

Review URL: https://codereview.chromium.org/421553002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22646 ce2b1a6d-e550-0410-aec6-3dcde31c8c00