Adjust to checkout_opts -> checkout_options
Remove custom checkout opts init function
Move checkout functions options more in line with the library

Afjust Checkout -> CheckoutHead and pass nil if the options structure is
nil so as not to overide the library's decisions.
CheckoutIndex: allow for index to be nil

Allow for the index to be nil and pass that to the library to use the
repository's index.
Merge branch 'cmn/checkout-opts'