grep: protect packed_git [re-]initialization

Some fields in struct raw_object_store are lazy initialized by the thread-unsafe packfile.c:prepare_packed_git(). Although this function is present in the call stack of git-grep threads, all paths to it are currently protected by obj_read_lock() (and the main thread usually indirectly calls it before firing the worker threads, anyway). However, it's possible that future modifications add new unprotected paths to it, introducing a race condition. Because errors derived from it wouldn't happen often, it could be hard to detect. So to prevent future headaches, let's force eager initialization of packed_git when setting git-grep up. There'll be a small overhead in the cases where we didn't really need to prepare packed_git during execution but this shouldn't be very noticeable. Also, packed_git may be re-initialized by packfile.c:reprepare_packed_git(). Again, all paths to it in git-grep are already protected by obj_read_lock() but it may suffer from the same problem in the future. So let's also internally protect it with obj_read_lock() (which is a recursive mutex). Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br> Signed-off-by: Junio C Hamano <gitster@pobox.com>