grep: take the read-lock when adding a submodule

With --recurse-submodules, we add each submodule that we encounter to the list of alternate object databases. With threading, our changes to the list are not protected against races. Indeed, ThreadSanitizer reports a race when we call `add_to_alternates_memory()` around the same time that another thread is reading in the list through `read_sha1_file()`. Take the grep read-lock while adding the submodule. The lock is used to serialize uses of non-thread-safe parts of Git's API, including `read_sha1_file()`. Helped-by: Brandon Williams <bmwill@google.com> Signed-off-by: Martin Ågren <martin.agren@gmail.com> Acked-by: Brandon Williams <bmwill@google.com> Reviewed-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>