run_external_diff: use an argv_array for the command line

We currently generate the command-line for the external
command using a fixed-length array of size 10. But if there
is a rename, we actually need 11 elements (10 items, plus a
NULL), and end up writing a random NULL onto the stack.

Rather than bump the limit, let's just use an argv_array, which
makes this sort of error impossible.
Noticed-by: NMax L <infthi.inbox@gmail.com>
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

diff.c

@@ -16,6 +16,7 @@
 #include "submodule.h"
 #include "ll-merge.h"
 #include "string-list.h"
+#include "argv-array.h"
 
 #ifdef NO_FAST_WORKING_DIRECTORY
 #define FAST_WORKING_DIRECTORY 0
@@ -2906,9 +2907,8 @@ static void run_external_diff(const char *pgm,
 			      int complete_rewrite,
 			      struct diff_options *o)
 {
-	const char *spawn_arg[10];
+	struct argv_array argv = ARGV_ARRAY_INIT;
 	int retval;
-	const char **arg = &spawn_arg[0];
 	struct diff_queue_struct *q = &diff_queued_diff;
 	const char *env[3] = { NULL };
 	char env_counter[50];
@@ -2919,23 +2919,22 @@ static void run_external_diff(const char *pgm,
 		const char *othername = (other ? other : name);
 		temp_one = prepare_temp_file(name, one);
 		temp_two = prepare_temp_file(othername, two);
-		*arg++ = pgm;
-		*arg++ = name;
-		*arg++ = temp_one->name;
-		*arg++ = temp_one->hex;
-		*arg++ = temp_one->mode;
-		*arg++ = temp_two->name;
-		*arg++ = temp_two->hex;
-		*arg++ = temp_two->mode;
+		argv_array_push(&argv, pgm);
+		argv_array_push(&argv, name);
+		argv_array_push(&argv, temp_one->name);
+		argv_array_push(&argv, temp_one->hex);
+		argv_array_push(&argv, temp_one->mode);
+		argv_array_push(&argv, temp_two->name);
+		argv_array_push(&argv, temp_two->hex);
+		argv_array_push(&argv, temp_two->mode);
 		if (other) {
-			*arg++ = other;
-			*arg++ = xfrm_msg;
+			argv_array_push(&argv, other);
+			argv_array_push(&argv, xfrm_msg);
 		}
 	} else {
-		*arg++ = pgm;
-		*arg++ = name;
+		argv_array_push(&argv, pgm);
+		argv_array_push(&argv, name);
 	}
-	*arg = NULL;
 	fflush(NULL);
 
 	env[0] = env_counter;
@@ -2944,8 +2943,9 @@ static void run_external_diff(const char *pgm,
 	env[1] = env_total;
 	snprintf(env_total, sizeof(env_total), "GIT_DIFF_PATH_TOTAL=%d", q->nr);
 
-	retval = run_command_v_opt_cd_env(spawn_arg, RUN_USING_SHELL, NULL, env);
+	retval = run_command_v_opt_cd_env(argv.argv, RUN_USING_SHELL, NULL, env);
 	remove_tempfile();
+	argv_array_clear(&argv);
 	if (retval) {
 		fprintf(stderr, "external diff died, stopping at %s.\n", name);
 		exit(1);