cmd editing: optimize/shrink output blanking

No need to output spaces 1 char at a time in a loop when the printf code
can do the same thing with the right format string.  This shrinks things
and gives a nice speed up when killing off lines more than a byte or two
as printf will send out the buffer in one big chunk.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>