Use warning function instead of fprintf(stderr, "Warning: ...").

Signed-off-by: Thiago Farina <tfransosi@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bisect.c

@@ -813,11 +813,11 @@ static void handle_skipped_merge_base(const unsigned char *mb)
 	char *bad_hex = sha1_to_hex(current_bad_sha1);
 	char *good_hex = join_sha1_array_hex(&good_revs, ' ');
 
-	fprintf(stderr, "Warning: the merge base between %s and [%s] "
+	warning("the merge base between %s and [%s] "
 		"must be skipped.\n"
 		"So we cannot be sure the first bad commit is "
 		"between %s and %s.\n"
-		"We continue anyway.\n",
+		"We continue anyway.",
 		bad_hex, good_hex, mb_hex, bad_hex);
 	free(good_hex);
 }