name-rev: pre-size buffer in get_parent_name()

We can calculate the size of new name easily and precisely. Open-code
the xstrfmt() calls and grow the buffers as needed before filling them.
This provides a surprisingly large benefit when working with the
Chromium repository; here are the numbers measured using hyperfine
before:

Benchmark #1: ./git -C ../chromium/src name-rev --all
  Time (mean ± σ):      5.822 s ±  0.013 s    [User: 5.304 s, System: 0.516 s]
  Range (min … max):    5.803 s …  5.837 s    10 runs

... and with this patch:

Benchmark #1: ./git -C ../chromium/src name-rev --all
  Time (mean ± σ):      1.527 s ±  0.003 s    [User: 1.015 s, System: 0.511 s]
  Range (min … max):    1.524 s …  1.535 s    10 runs

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/name-rev.c

@@ -103,15 +103,23 @@ static struct rev_name *create_or_update_name(struct commit *commit,
 
 static char *get_parent_name(const struct rev_name *name, int parent_number)
 {
+	struct strbuf sb = STRBUF_INIT;
 	size_t len;
 
 	strip_suffix(name->tip_name, "^0", &len);
-	if (name->generation > 0)
-		return xstrfmt("%.*s~%d^%d", (int)len, name->tip_name,
-			       name->generation, parent_number);
-	else
-		return xstrfmt("%.*s^%d", (int)len, name->tip_name,
-			       parent_number);
+	if (name->generation > 0) {
+		strbuf_grow(&sb, len +
+			    1 + decimal_width(name->generation) +
+			    1 + decimal_width(parent_number));
+		strbuf_addf(&sb, "%.*s~%d^%d", (int)len, name->tip_name,
+			    name->generation, parent_number);
+	} else {
+		strbuf_grow(&sb, len +
+			    1 + decimal_width(parent_number));
+		strbuf_addf(&sb, "%.*s^%d", (int)len, name->tip_name,
+			    parent_number);
+	}
+	return strbuf_detach(&sb, NULL);
 }
 
 static void name_rev(struct commit *start_commit,