refs: do not check ref existence in `is_root_ref()`

Before this patch series, root refs except for "HEAD" and our special refs were classified as pseudorefs. Furthermore, our terminology clarified that pseudorefs must not be symbolic refs. This restriction is enforced in `is_root_ref()`, which explicitly checks that a supposed root ref resolves to an object ID without recursing. This has been extremely confusing right from the start because (in old terminology) a ref name may sometimes be a pseudoref and sometimes not depending on whether it is a symbolic or regular ref. This behaviour does not seem reasonable at all and I very much doubt that it results in anything sane. Last but not least, the current behaviour can actually lead to a segfault when calling `is_root_ref()` with a reference that either does not exist or that is a symbolic ref because we never initialized `oid`, but then read it via `is_null_oid()`. We have now changed terminology to clarify that pseudorefs are really only "MERGE_HEAD" and "FETCH_HEAD", whereas all the other refs that live in the root of the ref hierarchy are just plain refs. Thus, we do not need to check whether the ref is symbolic or not. In fact, we can now avoid looking up the ref completely as the name is sufficient for us to figure out whether something would be a root ref or not. This change of course changes semantics for our callers. As there are only three of them we can assess each of them individually: - "ref-filter.c:ref_kind_from_refname()" uses it to classify refs. It's clear that the intent is to classify based on the ref name, only. - "refs/reftable_backend.c:reftable_ref_iterator_advance()" uses it to filter root refs. Again, using existence checks is pointless here as the iterator has just surfaced the ref, so we know it does exist. - "refs/files_backend.c:add_pseudoref_and_head_entries()" uses it to determine whether it should add a ref to the root directory of its iterator. This had the effect that we skipped over any files that are either a symbolic ref, or which are not a ref at all. The new behaviour is to include symbolic refs know, which aligns us with the adapted terminology. Furthermore, files which look like root refs but aren't are now mark those as "broken". As broken refs are not surfaced by our tooling, this should not lead to a change in user-visible behaviour, but may cause us to emit warnings. This feels like the right thing to do as we would otherwise just silently ignore corrupted root refs completely. So in all cases the existence check was either superfluous, not in line with the adapted terminology or masked potential issues. This commit thus changes the behaviour as proposed and drops the existence check altogether. Add a test that verifies that this does not change user-visible behaviour. Namely, we still don't want to show broken refs to the user by default in git-for-each-ref(1). What this does allow though is for internal callers to surface dangling root refs when they pass in the `DO_FOR_EACH_INCLUDE_BROKEN` flag. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>