fetch: fix failed batched updates skipping operations

Fix a regression introduced with batched updates in 0e358de64a (fetch:
use batched reference updates, 2025-05-19) when fetching references. In
the `do_fetch()` function, we jump to cleanup if committing the
transaction fails, regardless of whether using batched or atomic
updates. This skips three subsequent operations:

  - Update 'FETCH_HEAD' as part of `commit_fetch_head()`.

  - Add upstream tracking information via `set_upstream()`.

  - Setting remote 'HEAD' values when `do_set_head` is true.

For atomic updates, this is expected behavior. For batched updates,
we want to continue with these operations even if some refs fail to
update.

Skipping `commit_fetch_head()` isn't actually a regression because
'FETCH_HEAD' is already updated via `append_fetch_head()` when not
using '--atomic'. However, we add a test to validate this behavior.

Skipping the other two operations (upstream tracking and remote HEAD)
is a regression. Fix this by only jumping to cleanup when using
'--atomic', allowing batched updates to continue with post-fetch
operations. Add tests to prevent future regressions.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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