Use .git/config for storing "origin" shortcut repository

Use .git/config for storing "origin" shortcut repository

Rather than use a separate config .git/remotes/ for remote shortcuts, this
patch adds the analagous definitions to .git/config using git-repo-config
calls.

For example what was previously .git/remotes/origin
  URL: proto://host/path
  Pull: refs/heads/master:refs/heads/origin
Is now added to .git/config as
  [remote "origin"]
  url = proto://host/path
  fetch = refs/heads/master:refs/heads/origin

Signed-off-by: Andy Parkins <andyparkins@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>