Merge branch 'bw/grep-recurse-submodules' into maint

A broken access to object databases in recent update to "git grep
--recurse-submodules" has been fixed.

* bw/grep-recurse-submodules:
  grep: take the read-lock when adding a submodule

builtin/grep.c

@@ -431,7 +431,9 @@ static int grep_submodule(struct grep_opt *opt, struct repository *superproject,
 	 * store is no longer global and instead is a member of the repository
 	 * object.
 	 */
+	grep_read_lock();
 	add_to_alternates_memory(submodule.objectdir);
+	grep_read_unlock();
 
 	if (oid) {
 		struct object *object;