Implement "nogit clone"

This is logically just a git clone with the following two differences:

  1. It works in the current directory instead of making a new directory

  2. It puts its object store into .nogit instead of .git, (and it's just
     fine if there's already a .git here).

There are safety checks and it will refuse to clone in either of the
following cases:

  1. There is already a .nogit directory present

  2. Any of the working-tree files to be cloned already exist