pack-objects: move read mutex to packing_data struct

ac77d0c37 ("pack-objects: shrink size field in struct object_entry",
2018-04-14) added an extra usage of read_lock/read_unlock in the newly
introduced oe_get_size_slow for thread safety in parallel calls to
try_delta(). Unfortunately oe_get_size_slow is also used in serial
code, some of which is called before the first invocation of
ll_find_deltas. As such the read mutex is not guaranteed to be
initialized.

Resolve this by moving the read mutex to packing_data and initializing
it in prepare_packing_data which is initialized in cmd_pack_objects.

Signed-off-by: Patrick Hogg <phogg@novamoon.net>
Reviewed-by: Duy Nguyen <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-objects.c

@@ -150,6 +150,7 @@ void prepare_packing_data(struct packing_data *pdata)
 						   1UL << OE_DELTA_SIZE_BITS);
 #ifndef NO_PTHREADS
 	pthread_mutex_init(&pdata->lock, NULL);
+	init_recursive_mutex(&pdata->read_lock);
 #endif
 }