In some cases, resolving references requires us to also know about the
final symbolic reference that's pointing to a nonexistent branch, e.g.
in an empty repository where the main branch is yet unborn but HEAD
already points to it. Right now, the resolving logic is thus split up
into two, where one is the new refdb implementation and the second one
is an ad-hoc implementation inside "refs.c".

Let's extend `git_refdb_resolve` to also return such final dangling
references pointing to nonexistent branches so we can deduplicate the
resolving logic.