refs: stop resolving ref corresponding to reflogs

refs: stop resolving ref corresponding to reflogs

The reflog iterator tries to resolve the corresponding ref for every
reflog that it is about to yield. Historically, this was done due to
multiple reasons:

  - It ensures that the refname is safe because we end up calling
    `check_refname_format()`. Also, non-conformant refnames are skipped
    altogether.

  - The iterator used to yield the resolved object ID as well as its
    flags to the callback. This info was never used though, and the
    corresponding parameters were dropped in the preceding commit.

  - When a ref is corrupt then the reflog is not emitted at all.

We're about to introduce a new `git reflog list` subcommand that will
print all reflogs that the refdb knows about. Skipping over reflogs
whose refs are corrupted would be quite counterproductive in this case
as the user would have no way to learn about reflogs which may still
exist in their repository to help and rescue such a corrupted ref. Thus,
the only remaining reason for why we'd want to resolve the ref is to
verify its refname.

Refactor the code to call `check_refname_format()` directly instead of
trying to resolve the ref. This is significantly more efficient given
that we don't have to hit the object database anymore to list reflogs.
And second, it ensures that we end up showing reflogs of broken refs,
which will help to make the reflog more useful.

Note that this really only impacts the case where the corresponding ref
is corrupt. Reflogs for nonexistent refs would have been returned to the
caller beforehand already as we did not pass `RESOLVE_REF_READING` to
the function, and thus `refs_resolve_ref_unsafe()` would have returned
successfully in that case.

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