update-ref: move transaction handling into `update_refs_stdin()`

update-ref: move transaction handling into `update_refs_stdin()`

While the actual logic to update the transaction is handled in
`update_refs_stdin()`, the transaction itself is started and committed
in `cmd_update_ref()` itself. This makes it hard to handle transaction
abortion and commits as part of `update_refs_stdin()` itself, which is
required in order to introduce transaction handling features to `git
update-refs --stdin`.

Refactor the code to move all transaction handling into
`update_refs_stdin()` to prepare for transaction handling features.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>