diff: drop line_prefix_length field

diff: drop line_prefix_length field

The diff_options structure holds a line_prefix string and an associated
length. But the length is always just the strlen() of the NUL-terminated
string. Let's simplify the code by just storing the string pointer and
assuming it is NUL-terminated when we use it.

This will cause us to compute the string length in a few extra spots,
but I don't think any of these are particularly hot code paths.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>