resolve tree information from url in git sources

Git urls in sources currently support an irregular "https://github.com/user/repo/path" signature, with branch nam expected in the value key. A more conventional format would be "https://github.com/user/repo/tree/branch/path", with the value being the entry array directly, and branch name parsed from the url just like the path for parse-checkout. 
This should be a minor change. 