git: protect against unbalanced calls to {save,restore}_env()

We made sure that save_env_before_alias() does not skip saving the
environment when asked to (which led to use-after-free of orig_cwd
in restore_env() in the buggy version) with the previous step.

Protect against future breakage where somebody adds new callers of
these functions in an unbalanced fashion.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git.c

@@ -26,11 +26,15 @@ static const char *env_names[] = {
 };
 static char *orig_env[4];
 static int saved_env_before_alias;
+static int save_restore_env_balance;
 
 static void save_env_before_alias(void)
 {
 	int i;
 	saved_env_before_alias = 1;
+
+	assert(save_restore_env_balance == 0);
+	save_restore_env_balance = 1;
 	orig_cwd = xgetcwd();
 	for (i = 0; i < ARRAY_SIZE(env_names); i++) {
 		orig_env[i] = getenv(env_names[i]);
@@ -42,6 +46,9 @@ static void save_env_before_alias(void)
 static void restore_env(int external_alias)
 {
 	int i;
+
+	assert(save_restore_env_balance == 1);
+	save_restore_env_balance = 0;
 	if (!external_alias && orig_cwd && chdir(orig_cwd))
 		die_errno("could not move to %s", orig_cwd);
 	free(orig_cwd);