submodule--helper: don't leak {run,capture}_command() cp.dir argument

Fix a memory leak in c51f8f94e5 (submodule--helper: run update
procedures from C, 2021-08-24) and 3c3558f095 (submodule--helper: run
update using child process struct, 2022-03-15) by not allocating
memory in the first place.

The "dir" member of "struct child_process" will not be modified by
that API, and it's declared to be "const char *". So let's not
needlessly duplicate these strings.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Glen Choo <chooglen@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/submodule--helper.c

@@ -2126,7 +2126,7 @@ static int is_tip_reachable(const char *path, const struct object_id *oid)
 	char *hex = oid_to_hex(oid);
 
 	cp.git_cmd = 1;
-	cp.dir = xstrdup(path);
+	cp.dir = path;
 	cp.no_stderr = 1;
 	strvec_pushl(&cp.args, "rev-list", "-n", "1", hex, "--not", "--all", NULL);
 
@@ -2145,7 +2145,7 @@ static int fetch_in_submodule(const char *module_path, int depth, int quiet,
 
 	prepare_submodule_repo_env(&cp.env);
 	cp.git_cmd = 1;
-	cp.dir = xstrdup(module_path);
+	cp.dir = module_path;
 
 	strvec_push(&cp.args, "fetch");
 	if (quiet)
@@ -2198,7 +2198,7 @@ static int run_update_command(const struct update_data *ud, int subforce)
 	}
 	strvec_push(&cp.args, oid);
 
-	cp.dir = xstrdup(ud->sm_path);
+	cp.dir = ud->sm_path;
 	prepare_submodule_repo_env(&cp.env);
 	if ((ret = run_command(&cp))) {
 		switch (ud->update_strategy.type) {