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>

diff.h

@@ -274,7 +274,6 @@ struct diff_options {
 	const char *single_follow;
 	const char *a_prefix, *b_prefix;
 	const char *line_prefix;
-	size_t line_prefix_length;
 
 	/**
 	 * collection of boolean options that affects the operation, but some do