builtin/maintenance: fix leaking config string

When parsing the maintenance strategy from config we allocate a config
string, but do not free it after parsing it. Plug this leak by instead
using `git_config_get_string_tmp()`, which does not allocate any memory.

This leak is exposed by t7900, but plugging it alone does not make the
test suite pass.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

builtin/gc.c

@@ -1476,9 +1476,9 @@ static int maintenance_run_tasks(struct maintenance_run_opts *opts,
 
 static void initialize_maintenance_strategy(void)
 {
-	char *config_str;
+	const char *config_str;
 
-	if (git_config_get_string("maintenance.strategy", &config_str))
+	if (git_config_get_string_tmp("maintenance.strategy", &config_str))
 		return;
 
 	if (!strcasecmp(config_str, "incremental")) {