Set the remote config manually

See 5cd6b35 and 8eefd4e.

Library/Homebrew/cmd/update.rb

@@ -79,7 +79,8 @@ module Homebrew
     if Dir[".git/*"].empty?
       safe_system "git", "init"
       safe_system "git", "config", "core.autocrlf", "false"
-      safe_system "git", "remote", "add", "origin", "https://github.com/Homebrew/homebrew.git"
+      safe_system "git", "config", "remote.origin.url", "https://github.com/Homebrew/homebrew.git"
+      safe_system "git", "config", "remote.origin.fetch", "+refs/heads/*:refs/remotes/origin/*"
       safe_system "git", "fetch", "origin"
       safe_system "git", "reset", "--hard", "origin/master"
     end