submodule--helper: fix a leak with repo_clear()

Call repo_clear() in ensure_core_worktree() to free the "struct
repository". Fixes a leak that's been here since
74d4731da1 (submodule--helper: replace connect-gitdir-workingtree by
ensure-core-worktree, 2018-08-13).

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

@@ -2397,6 +2397,7 @@ static int ensure_core_worktree(const char *path)
 		strbuf_release(&sb);
 	}
 
+	repo_clear(&subrepo);
 	return 0;
 }