Fix install script to remember --git-name and --git-email

[mcmire]

~/.gitconfig is different because it's actually copied — a symlink doesn't get created. This is because we need to run git config --global user.name <name> and git config --global user.email <email>. The problem is that if you want to update your git config, simply running bin/manage install won't apply those updates, as then, ~/.gitconfig would get overwritten. This wouldn't be a problem if the manage script somehow reused your current git name and email.

[mcmire]

#13 may obsolete this.