clean: improve -n and -f implementation and documentation
What -n actually does in addition to its documented behavior is ignoring of configuration variable clean.requireForce, that makes sense provided -n prevents files removal anyway. So, first, document this in the manual, and then modify implementation to make this more explicit in the code. Improved implementation also stops to share single internal variable 'force' between command-line -f option and configuration variable clean.requireForce, resulting in more clear logic. Two error messages with slightly different text depending on if clean.requireForce was explicitly set or not, are merged into a single one. The resulting error message now does not mention -n as well, as it neither matches intended clean.requireForce usage nor reflects clarified implementation. Documentation of clean.requireForce is changed accordingly. Signed-off-by: Sergey Organov <sorganov@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>