odb: move initialization bit into struct packfile_store

The object database knows to skip re-initializing the list of packfiles
in case it's already been initialized. Whether or not that is the case
is tracked via a separate `initialized` bit that is stored in the object
database. With the introduction of the `struct packfile_store` we have a
better place to host this bit though.

Move it accordingly. While at it, convert the field into a boolean now
that we're allowed to use them in our code base.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

packfile.h

@@ -63,6 +63,12 @@ struct packfile_store {
 	 * the store.
 	 */
 	struct packed_git *packs;
+
+	/*
+	 * Whether packfiles have already been populated with this store's
+	 * packs.
+	 */
+	bool initialized;
 };
 
 /*