refs/files: batch refname availability checks for normal transactions

refs/files: batch refname availability checks for normal transactions

Same as the "reftable" backend that we have adapted in the preceding
commit to use batched refname availability checks we can also do so for
the "files" backend. Things are a bit more intricate here though, as we
call `refs_verify_refname_available()` in a set of different contexts:

  1. `lock_raw_ref()` when it hits either EEXISTS or EISDIR when creating
     a new reference, mostly to create a nice, user-readable error
     message. This is nothing we have to care about too much, as we only
     hit this code path at most once when we hit a conflict.

  2. `lock_raw_ref()` when it _could_ create the lockfile to check
     whether it is conflicting with any packed refs. In the general case,
     this code path will be hit once for every (successful) reference
     update.

  3. `lock_ref_oid_basic()`, but it is only executed when copying or
     renaming references or when expiring reflogs. It will thus not be
     called in contexts where we have many references queued up.

  4. `refs_refname_ref_available()`, but again only when copying or
     renaming references. It is thus not interesting due to the same
     reason as the previous case.

  5. `files_transaction_finish_initial()`, which is only executed when
     creating a new repository or migrating references.

So out of these, only (2) and (5) are viable candidates to use the
batched checks.

Adapt `lock_raw_ref()` accordingly by queueing up reference names that
need to be checked for availability and then checking them after we have
processed all updates. This check is done before we (optionally) lock
the `packed-refs` file, which is somewhat flawed because it means that
the `packed-refs` could still change after the availability check and
thus create an undetected conflict. But unconditionally locking the file
would change semantics that users are likely to rely on, so we keep the
current locking sequence intact, even if it's suboptmial.

The refactoring of `files_transaction_finish_initial()` will be done in
the next commit.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>