git-prompt: stop manually parsing HEAD with unknown ref formats

We're manually parsing the HEAD reference in git-prompt to figure out whether it is a symbolic or direct reference. This makes it intimately tied to the on-disk format we use to store references and will stop working once we gain additional reference backends in the Git project. Ideally, we would refactor the code to exclusively use plumbing tools to read refs such that we do not have to care about the on-disk format at all. Unfortunately though, spawning processes can be quite expensive on some systems like Windows. As the Git prompt logic may be executed quite frequently we try very hard to spawn as few processes as possible. This refactoring is thus out of question for now. Instead, condition the logic on the repository's ref format: if the repo uses the the "files" backend we can continue to use the old logic and read the respective files from disk directly. If it's anything else, then we use git-symbolic-ref(1) to read the value of HEAD. This change makes the Git prompt compatible with the upcoming "reftable" format. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>