Add tooling to list all refs

While git-for-each-ref(1) is able to list all refs in refs/, this does not include all references. The following references will not be listed by it:

• Special refs used by the sequencer like rebase-apply/ and rebase-merge/.
• Pseudo-refs like HEAD, ORIG_HEAD, MERGE_HEAD
• Anything else stored outside of the usual hierarchy.

It's actually not that easy to list all of these in the files backend, because any file that contains ref-like content kind of is a ref. But with the reftable backend it becomes a lot more important to be able to list everything contained in the database, which isn't easily possible right now.

We should think about a way to address this.