refs: classify HEAD as a root ref

Root refs are those refs that live in the root of the ref hierarchy. Our old and venerable "HEAD" reference falls into this category, but we don't yet classify it as such in `is_root_ref()`. Adapt the function to also treat "HEAD" as a root ref. This change is safe to do for all current callers: - `ref_kind_from_refname()` already handles "HEAD" explicitly before calling `is_root_ref()`. - The "files" and "reftable" backends explicitly call both `is_root_ref()` and `is_headref()` together. This also aligns behaviour or `is_root_ref()` and `is_headref()` such that we stop checking for ref existence. This changes semantics for our backends: - In the reftable backend we already know that the ref must exist because `is_headref()` is called as part of the ref iterator. The existence check is thus redundant, and the change is safe to do. - In the files backend we use it when populating root refs, where we would skip adding the "HEAD" file if it was not possible to resolve it. The new behaviour is to instead mark "HEAD" as broken, which will cause us to emit warnings in various places. As there are no callers of `is_headref()` left afer the refactoring, we can absorb it completely into `is_root_ref()`. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>