refs: implement batch reference update support

Git supports making reference updates with or without transactions.
Updates with transactions are generally better optimized. But
transactions are all or nothing. This means, if a user wants to batch
updates to take advantage of the optimizations without the hard
requirement that all updates must succeed, there is no way currently to
do so. Particularly with the reftable backend where batching multiple
reference updates is more efficient than performing them sequentially.

Introduce batched update support with a new flag,
'REF_TRANSACTION_ALLOW_FAILURE'. Batched updates while different from
transactions, use the transaction infrastructure under the hood. When
enabled, this flag allows individual reference updates that would
typically cause the entire transaction to fail due to non-system-related
errors to be marked as rejected while permitting other updates to
proceed. System errors referred by 'REF_TRANSACTION_ERROR_GENERIC'
continue to result in the entire transaction failing. This approach
enhances flexibility while preserving transactional integrity where
necessary.

The implementation introduces several key components:

  - Add 'rejection_err' field to struct `ref_update` to track failed
    updates with failure reason.

  - Add a new struct `ref_transaction_rejections` and a field within
    `ref_transaction` to this struct to allow quick iteration over
    rejected updates.

  - Modify reference backends (files, packed, reftable) to handle
    partial transactions by using `ref_transaction_set_rejected()`
    instead of failing the entire transaction when
    `REF_TRANSACTION_ALLOW_FAILURE` is set.

  - Add `ref_transaction_for_each_rejected_update()` to let callers
    examine which updates were rejected and why.

This foundational change enables batched update support throughout the
reference subsystem. A following commit will expose this capability to
users by adding a `--batch-updates` flag to 'git-update-ref(1)',
providing both a user-facing feature and a testable implementation.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Acked-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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