pack-objects (mingw): initialize packing_data mutex in the correct spot

In 9ac3f0e5b3 (pack-objects: fix performance issues on packing large
deltas, 2018-07-22), a mutex was introduced that is used to guard the
call to set the delta size. This commit even added code to initialize
it, but at an incorrect spot: in `init_threaded_search()`, while the
call to `oe_set_delta_size()` (and hence to `packing_data_lock()`) can
happen in the call chain `check_object()` <- `get_object_details()` <-
`prepare_pack()` <- `cmd_pack_objects()`, which is long before the
`prepare_pack()` function calls `ll_find_deltas()` (which initializes
the threaded search).

Another tell-tale that the mutex was initialized in an incorrect spot is
that the function to initialize it lives in builtin/, while the code
that uses the mutex is defined in a libgit.a header file.

Let's use a more appropriate function: `prepare_packing_data()`, which
not only lives in libgit.a, but *has* to be called before the
`packing_data` struct is used that contains that mutex.

This fixes https://github.com/git-for-windows/git/issues/1839.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack-objects.c

@@ -148,6 +148,9 @@ void prepare_packing_data(struct packing_data *pdata)
 					     1U << OE_SIZE_BITS);
 	pdata->oe_delta_size_limit = git_env_ulong("GIT_TEST_OE_DELTA_SIZE",
 						   1UL << OE_DELTA_SIZE_BITS);
+#ifndef NO_PTHREADS
+	pthread_mutex_init(&pdata->lock, NULL);
+#endif
 }
 
 struct object_entry *packlist_alloc(struct packing_data *pdata,