utf8.c: speculatively assume utf-8 in strbuf_add_wrapped_text()

is_utf8() works by calling utf8_width() for each character at the
supplied location.  In strbuf_add_wrapped_text(), we do that anyway
while wrapping the lines.  So instead of checking the encoding
beforehand, optimistically assume that it's utf-8 and wrap along
until an invalid character is hit, and when that happens start over.

This pays off if the text consists only of valid utf-8 characters.
The following command was run against the Linux kernel repo with
git 1.7.0:

	$ time git log --format='%b' v2.6.32 >/dev/null

	real	0m2.679s
	user	0m2.580s
	sys	0m0.100s

	$ time git log --format='%w(60,4,8)%b' >/dev/null

	real	0m4.342s
	user	0m4.230s
	sys	0m0.110s

And with this patch series:

	$ time git log --format='%w(60,4,8)%b' >/dev/null

	real	0m3.741s
	user	0m3.630s
	sys	0m0.110s

So the cost of wrapping is reduced to 70% in this case.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

utf8.c

@@ -324,16 +324,21 @@ static size_t display_mode_esc_sequence_len(const char *s)
  * consumed (and no extra indent is necessary for the first line).
  */
 int strbuf_add_wrapped_text(struct strbuf *buf,
-		const char *text, int indent, int indent2, int width)
+		const char *text, int indent1, int indent2, int width)
 {
-	int w = indent, assume_utf8 = is_utf8(text);
+	int indent, w, assume_utf8 = 1;
-	const char *bol = text, *space = NULL;
+	const char *bol, *space, *start = text;
+	size_t orig_len = buf->len;
 
 	if (width <= 0) {
-		strbuf_add_indented_text(buf, text, indent, indent2);
+		strbuf_add_indented_text(buf, text, indent1, indent2);
 		return 1;
 	}
 
+retry:
+	bol = text;
+	w = indent = indent1;
+	space = NULL;
 	if (indent < 0) {
 		w = -indent;
 		space = text;
@@ -385,9 +390,15 @@ new_line:
 			}
 			continue;
 		}
-		if (assume_utf8)
+		if (assume_utf8) {
 			w += utf8_width(&text, NULL);
-		else {
+			if (!text) {
+				assume_utf8 = 0;
+				text = start;
+				strbuf_setlen(buf, orig_len);
+				goto retry;
+			}
+		} else {
 			w++;
 			text++;
 		}