Make url accesses retry. Git retry requires some more analysis of git output first (follow up CL).

BUG=
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/91733003

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@18174 ce2b1a6d-e550-0410-aec6-3dcde31c8c00