Stop deleting people's root filesystems, disable user heuristics by default, and add git to username blacklist

When we improperly guess that a system user is a human user, enticing actual human users to delete the mysterious unknown user account in gnome-control-center, very bad things happen if the human decides to delete the mysterious user's home directory and that home directory is the root filesystem, as is the case for lots of system users.

With apologies to all the Arch Linux users who fell for this trap and deleted their filesystems.