sparse-checkout: pay attention to prefix for {set, add}

In cone mode, non-option arguments to set & add are clearly paths, and
as such, we should pay attention to prefix.

In non-cone mode, it is not clear that folks intend to provide paths
since the inputs are gitignore-style patterns.  Paying attention to
prefix would prevent folks from doing things like
   git sparse-checkout add /.gitattributes
   git sparse-checkout add '/toplevel-dir/*'
In fact, the former will result in
   fatal: '/.gitattributes' is outside repository...
while the later will result in
   fatal: Invalid path '/toplevel-dir': No such file or directory
despite the fact that both are valid gitignore-style patterns that would
select real files if added to the sparse-checkout file.  This might lead
people to just use the path without the leading slash, potentially
resulting in them grabbing files with the same name throughout the
directory hierarchy contrary to their expectations.  See also [1] and
[2].  Adding prefix seems to just be fraught with error; so for now
simply throw an error in non-cone mode when sparse-checkout set/add are
run from a subdirectory.

[1] https://lore.kernel.org/git/e1934710-e228-adc4-d37c-f706883bd27c@gmail.com/
[2] https://lore.kernel.org/git/CABPp-BHXZ-XLxY0a3wCATfdq=6-EjW62RzbxKAoFPeXfJswD2w@mail.gmail.com/

Helped-by: Junio Hamano <gitster@pobox.com>
Reviewed-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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