Hi. count_newlines is passed the wrong buffer length (off-by-one) so it
overwrites first differing byte in first file's buffer. Attached patch
fixes.

Actual:

$ cmp -b <(echo bad) <(echo bug)
[snip] differ: byte 2, line 1 is  12 ^J 165 u


Expected:

$ cmp -b <(echo bad) <(echo bug)
[snip] differ: byte 2, line 1 is 141 a 165 u

Cheers!

--mancha