Allow specifying --dirstat cut-off percentage as a floating point number
Only the first digit after the decimal point is kept, as the dirstat
calculations all happen in permille.
Selftests verifying floating-point percentage input has been added.
Improved-by: Junio C Hamano <gitster@pobox.com> Improved-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Johan Herland <johan@herland.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>