refs API: make files_copy_or_rename_ref() et al not set errno

refs API: make files_copy_or_rename_ref() et al not set errno

None of the callers of rename_ref() and copy_ref() care about errno,
and as seen in the context here we already emit our own non-errno
using error() in the case where we'd use it.

So let's have it explicitly ignore errno, and do the same in
commit_ref_update(), which is only used within other code in
files_copy_or_rename_ref() itself which doesn't care about errno
either.

It might actually be sensible to have the callers use errno if the
failure was filesystem-specific, and with the upcoming reftable
backend we don't want to rely on that sort of thing, so let's keep
ignoring that for now.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>