refs/reftable: extract code from the transaction preparation

Extract the core logic for preparing individual reference updates from
`reftable_be_transaction_prepare()` into `prepare_single_update()`. This
dedicated function now handles all validation and preparation steps for
each reference update in the transaction, including object ID
verification, HEAD reference handling, and symref processing.

The refactoring consolidates all reference update validation into a
single logical block, which improves code maintainability and
readability. More importantly, this restructuring lays the groundwork
for implementing partial transaction support in the reftable backend,
which will be introduced in the following commit.

No functional changes are included in this commit - it is purely a code
reorganization to support future enhancements.

Signed-off-by: Karthik Nayak <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: KarthikNayak