diff: retire sane_truncate_fn

Long time ago, 23707811 ("diff: do not chomp hunk-header in the
middle of a character", 2008-01-02) introduced sane_truncate_line()
helper function to trim the "function header" line that is shown at
the end of the hunk header line, in order to avoid chomping it in
the middle of a single UTF-8 character.  It also added a facility to
define a custom callback function to make it possible to extend it
to non UTF-8 encodings.

During the following 8 1/2 years, nobody found need for this custom
callback facility.

A custom callback function is a wrong design to use here anyway---if
your contents need support for non UTF-8 encoding, you shouldn't
have to write a custom function and recompile Git to plumb it in.  A
better approach would be to extend sane_truncate_line() function and
have a new member in emit_callback to conditionally trigger it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -406,8 +406,6 @@ static struct diff_tempfile {
 	struct tempfile tempfile;
 } diff_temp[2];
 
-typedef unsigned long (*sane_truncate_fn)(char *line, unsigned long len);
-
 struct emit_callback {
 	int color_diff;
 	unsigned ws_rule;
@@ -415,7 +413,6 @@ struct emit_callback {
 	int blank_at_eof_in_postimage;
 	int lno_in_preimage;
 	int lno_in_postimage;
-	sane_truncate_fn truncate;
 	const char **label_path;
 	struct diff_words_data *diff_words;
 	struct diff_options *opt;
@@ -1243,8 +1240,6 @@ static unsigned long sane_truncate_line(struct emit_callback *ecb, char *line, u
 	unsigned long allot;
 	size_t l = len;
 
-	if (ecb->truncate)
-		return ecb->truncate(line, len);
 	cp = line;
 	allot = l;
 	while (0 < l) {