Allow git:remote to update remotes with set-url
Clean up create/update_git_remote
 - Extract out has_git_remote?
 - Do not check for .git. Git does this for us and will work from sub
   directories. Instead, check for $?.success?
 - Only show success message on success
Update git:remote command doc
Correct wording of remote update doc
Do not prompt for git remote update
Extract DEFAULT_REMOTE
Extract git_url method
Extract remote_name method
Merge pull request #1232 from heroku/git-remote-update

Allow git:remote to update remotes with set-url