refs/files: detect race when generating reflog entry for HEAD

When updating a reference that is being pointed to HEAD we don't only
write a reflog message for that particular reference, but also generate
one for HEAD. This logic is handled by `split_head_update()`, where we:

  1. Verify that the condition actually triggered. This is done by
     reading HEAD at the start of the transaction so that we can then
     check whether a given reference update refers to its target.

  2. Queue a new log-only update for HEAD in case it did.

But the logic is unfortunately not free of races, as we do not lock the
HEAD reference after we have read its target. This can lead to the
following two scenarios:

  - HEAD gets concurrently updated to point to one of the references we
    have already processed. This causes us not writing a reflog message
    even though we should have done so.

  - HEAD gets concurrently updated to no longer point to a reference
    anymore that we have already processed. This causes us to write a
    reflog message even though we should _not_ have done so.

Improve the situation by introducing a new `REF_LOG_VIA_SPLIT` flag that
is specific to the "files" backend. If set, we will double check that
the HEAD reference still points to the reference that we are creating
the reflog entry for after we have locked HEAD. Furthermore, instead of
manually resolving the old object ID of that entry, we now use the same
old state as for the parent update.

If we detect such a racy update we abort the transaction. This is a bit
heavy-handed: the user didn't even ask us to write a reflog update for
"HEAD", so it might be surprising if we abort the transaction. That
being said:

  - Normal users wouldn't typically hit this case as we only hit the
    relevant code when committing to a branch that is being pointed to
    by "HEAD" directly. Commands like git-commit(1) typically commit to
    "HEAD" itself though.

  - Scripted users that use git-update-ref(1) and related plumbing
    commands are unlikely to hit this case either, as they would have to
    update the pointed-to-branch at the same as "HEAD" is being updated,
    which is an exceedingly rare event.

The alternative would be to instead drop the log-only update completely,
but that would require more logic that is hard to verify without adding
infrastructure specific for such a test. So we rather do the pragmatic
thing and don't worry too much about an edge case that is very unlikely
to happen.

Unfortunately, this change only helps with the second race. We cannot
reliably plug the first race without locking the HEAD reference at the
start of the transaction. Locking HEAD unconditionally would effectively
serialize all writes though, and that doesn't seem like an option. Also,
double checking its value at the end of the transaction is not an option
either, as its target may have flip-flopped during the transaction.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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