Many code paths in checkout need the final, full on-disk path of the
file they're writing.  (No surprise).  However, they all munge the
`data->path` buffer themselves to get there.  Provide a nice helper
method for them.

Plus, drop the use `git_iterator_current_workdir_path` which does the
same thing but different.  Checkout is the only caller of this silly
function, which lets us remove it.