grep: allow submodule functions to run in parallel

Now that object reading operations are internally protected, the submodule initialization functions at builtin/grep.c:grep_submodule() are very close to being thread-safe. Let's take a look at each call and remove from the critical section what we can, for better performance: - submodule_from_path() and is_submodule_active() cannot be called in parallel yet only because they call repo_read_gitmodules() which contains, in its call stack, operations that would otherwise be in race condition with object reading (for example parse_object() and is_promisor_remote()). However, they only call repo_read_gitmodules() if it wasn't read before. So let's pre-read it before firing the threads and allow these two functions to safely be called in parallel. - repo_submodule_init() is already thread-safe, so remove it from the critical section without other necessary changes. - The repo_read_gitmodules(&subrepo) call at grep_submodule() is safe as no other thread is performing object reading operations in the subrepo yet. However, threads might be working in the superproject, and this function calls add_to_alternates_memory() internally, which is racy with object readings in the superproject. So it must be kept protected for now. Let's add a "NEEDSWORK" to it, informing why it cannot be removed from the critical section yet. - Finally, add_to_alternates_memory() must be kept protected for the same reason as the item above. Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br> Signed-off-by: Junio C Hamano <gitster@pobox.com>