refs/files: batch refname availability checks for initial transactions

The "files" backend explicitly carves out special logic for its initial transaction so that it can avoid writing out every single reference as a loose reference. While the assumption is that there shouldn't be any preexisting references, we still have to verify that none of the newly written references will conflict with any other new reference in the same transaction. Refactor the initial transaction to use batched refname availability checks. This does not yet have an effect on performance as we still call `refs_verify_refname_available()` in a loop. But this will change in subsequent commits and then impact performance when cloning a repository with many references or when migrating references to the "files" format. This will improve performance when cloning a repository with many references or when migrating references from any format to the "files" format once the availability checks have learned to optimize checks for many references in a subsequent commit. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>