rev: stop adding new line at the end when input does not have it

When the rev(1) is executed twice outcome is expected to be exactly what
it was originally.  That includes not adding new line at the end of the
output.  The oneliner below demonstrates earlier issue.

$ printf "abc\n123" | rev | rev

Signed-off-by: Sami Kerola <kerolasa@iki.fi>