go.tools/dashboard: clone main repo from local path if it already exists locally.
go.tools/dashboard: clone main repo from local path if it already exists locally.

R=adg, minux.ma
CC=golang-dev
https://codereview.appspot.com/14462049

skyportsystems/go.tools,skyportsystems/go.tools,bryanxu/go-zh.tools,skyportsystems/go.tools,8limbs/gotools,d0f/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,d0f/go-zh.tools,webfd/go-zh.tools,bryanxu/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,webfd/go-zh.tools,8limbs/gotools,skyportsystems/go.tools,8limbs/gotools,bryanxu/go-zh.tools,d0f/go-zh.tools,bryanxu/go-zh.tools,webfd/go-zh.tools,skyportsystems/go.tools