submodule--helper: fix leak of core.worktree value

In the ensure_core_worktree() function, we load the core.worktree value
of the submodule repository using repo_config_get_string(). This
function copies the string, but we never free it, leaking the memory.

We can instead use the "tmp" version of that function to avoid the
allocation at all. We don't have to worry about lifetime issues, since
we never even look at the value (we just want to know if it's set).

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

builtin/submodule--helper.c

@@ -2101,7 +2101,7 @@ static int ensure_core_worktree(int argc, const char **argv, const char *prefix)
 {
 	const struct submodule *sub;
 	const char *path;
-	char *cw;
+	const char *cw;
 	struct repository subrepo;
 
 	if (argc != 2)
@@ -2116,7 +2116,7 @@ static int ensure_core_worktree(int argc, const char **argv, const char *prefix)
 	if (repo_submodule_init(&subrepo, the_repository, sub))
 		die(_("could not get a repository handle for submodule '%s'"), path);
 
-	if (!repo_config_get_string(&subrepo, "core.worktree", &cw)) {
+	if (!repo_config_get_string_tmp(&subrepo, "core.worktree", &cw)) {
 		char *cfg_file, *abs_path;
 		const char *rel_path;
 		struct strbuf sb = STRBUF_INIT;