fetch: fix non-conflicting tags not being committed

fetch: fix non-conflicting tags not being committed

The commit 0e358de64a (fetch: use batched reference updates, 2025-05-19)
updated the 'git-fetch(1)' command to use batched updates. This batches
updates to gain performance improvements. When fetching references, each
update is added to the transaction. Finally, when committing, individual
updates are allowed to fail with reason, while the transaction itself
succeeds.

One scenario which was missed here, was fetching tags. When fetching
conflicting tags, the `fetch_and_consume_refs()` function returns '1',
which skipped committing the transaction and directly jumped to the
cleanup section. This mean that no updates were applied. This also
extends to backfilling tags which is done when fetching specific
refspecs which contains tags in their history.

Fix this by committing the transaction when we have an error code and
not using an atomic transaction. This ensures other references are
applied even when some updates fail.

The cleanup section is reached with `retcode` set in several scenarios:

   - `truncate_fetch_head()`, `open_fetch_head()` and `prune_refs()` set
     `retcode` before the transaction is created, so no commit is
     attempted.

   - `fetch_and_consume_refs()` and `backfill_tags()` are the primary
     cases this fix targets, both setting a positive `retcode` to
     trigger the committing of the transaction.

This simplifies error handling and ensures future modifications to
`do_fetch()` don't need special handling for batched updates.

Add tests to check for this regression. While here, add a missing
cleanup from previous test.

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