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>

Author: peff

diff.c

@@ -5395,7 +5395,6 @@ static int diff_opt_line_prefix(const struct option *opt,
 
 	BUG_ON_OPT_NEG(unset);
 	options->line_prefix = optarg;
-	options->line_prefix_length = strlen(options->line_prefix);
 	graph_setup_line_prefix(options);
 	return 0;
 }

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

graph.c

@@ -74,10 +74,7 @@ static void graph_show_line_prefix(const struct diff_options *diffopt)
 	if (!diffopt || !diffopt->line_prefix)
 		return;
 
-	fwrite(diffopt->line_prefix,
-	       sizeof(char),
-	       diffopt->line_prefix_length,
-	       diffopt->file);
+	fputs(diffopt->line_prefix, diffopt->file);
 }
 
 static const char **column_colors;
@@ -321,8 +318,7 @@ static struct strbuf *diff_output_prefix_callback(struct diff_options *opt, void
 
 	strbuf_reset(&msgbuf);
 	if (opt->line_prefix)
-		strbuf_add(&msgbuf, opt->line_prefix,
-			   opt->line_prefix_length);
+		strbuf_addstr(&msgbuf, opt->line_prefix);
 	if (graph)
 		graph_padding_line(graph, &msgbuf);
 	return &msgbuf;