run_external_diff: use an argv_array for the environment

We currently use static buffers and a static array for
formatting the environment passed to the external diff.
There's nothing wrong in the code, but it is much easier to
verify that it is correct if we use a dynamic argv_array.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.c

@@ -2908,11 +2908,9 @@ static void run_external_diff(const char *pgm,
 			      struct diff_options *o)
 {
 	struct argv_array argv = ARGV_ARRAY_INIT;
+	struct argv_array env = ARGV_ARRAY_INIT;
 	int retval;
 	struct diff_queue_struct *q = &diff_queued_diff;
-	const char *env[3] = { NULL };
-	char env_counter[50];
-	char env_total[50];
 
 	if (one && two) {
 		struct diff_tempfile *temp_one, *temp_two;
@@ -2937,15 +2935,13 @@ static void run_external_diff(const char *pgm,
 	}
 	fflush(NULL);
 
-	env[0] = env_counter;
-	snprintf(env_counter, sizeof(env_counter), "GIT_DIFF_PATH_COUNTER=%d",
-		 ++o->diff_path_counter);
-	env[1] = env_total;
-	snprintf(env_total, sizeof(env_total), "GIT_DIFF_PATH_TOTAL=%d", q->nr);
+	argv_array_pushf(&env, "GIT_DIFF_PATH_COUNTER=%d", ++o->diff_path_counter);
+	argv_array_pushf(&env, "GIT_DIFF_PATH_TOTAL=%d", q->nr);
 
-	retval = run_command_v_opt_cd_env(argv.argv, RUN_USING_SHELL, NULL, env);
+	retval = run_command_v_opt_cd_env(argv.argv, RUN_USING_SHELL, NULL, env.argv);
 	remove_tempfile();
 	argv_array_clear(&argv);
+	argv_array_clear(&env);
 	if (retval) {
 		fprintf(stderr, "external diff died, stopping at %s.\n", name);
 		exit(1);