directory: added inline wrappers for accessing children

Some tiny utilities... wrappers like these may become helpful when we
introduce locking.

src/directory.h

@@ -68,6 +68,20 @@ directory_get_path(struct directory *directory)
 	return directory->path;
 }
 
+static inline struct directory *
+directory_get_child(const struct directory *directory, const char *name)
+{
+	return dirvec_find(&directory->children, name);
+}
+
+static inline struct directory *
+directory_new_child(struct directory *directory, const char *name)
+{
+	struct directory *subdir = directory_new(name, directory);
+	dirvec_add(&directory->children, subdir);
+	return subdir;
+}
+
 void
 directory_prune_empty(struct directory *directory);