delta: widen `create_delta()` and `diff_delta()` to `size_t`

Last stop in the delta-encoding API widening for >4 GiB blobs on
Windows: with `create_delta_index()` done in the prior commit and
`create_delta()`/`diff_delta()` finished here, every byte count that
crosses delta.h is now `size_t`. The struct fields they store into have
been `size_t` since the diff-delta struct widening.

The API change must move with all callers in the same commit (the build
only passes when every `&delta_size` matches the new `size_t*`). Caller
updates are kept minimal:

  * builtin/pack-objects.c `get_delta()` and `try_delta()`: widen only
    the local `delta_size` variable; the surrounding unsigned-long
    locals and their `cast_size_t_to_ulong()` shims are out of scope
    here and will be cleaned up in their own commits.

  * builtin/fast-import.c, diff.c, t/helper/test-pack-deltas.c:
    keep the local unsigned-long delta size (each feeds a still-
    unsigned-long downstream consumer: zlib's `avail_in`,
    `deflate_it()`, the test helper's own `do_compress()`), and bridge
    via a temporary `size_t` plus `cast_size_t_to_ulong()`. The new
    casts are paid back in later topics that widen those consumers.

  * t/helper/test-delta.c: widen the local outright (no downstream
    consumer beyond the test's own `out_size`, which is already
    `size_t`).

Note that GCC struggles a bit to figure out that `deltalen` is always
initialized before it is used; To help it along, we initialize it to 0.
This work-around will go away in a later patch series when `deltalen`
can be widened to `size_t`.

Assisted-by: Opus 4.7
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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