refs/files: use correct error type when lock exists

refs/files: use correct error type when lock exists

When fetching references into a repository, if a lock for a particular
reference exists, then `lock_raw_ref()` throws:

    - REF_TRANSACTION_ERROR_CASE_CONFLICT: when there is a conflict
    because the transaction contains conflicting references while being
    on a case-insensitive filesystem.

    - REF_TRANSACTION_ERROR_GENERIC: for all other errors.

The latter causes the entire set of batched updates to fail, even in
case sensitive filessystems.

Instead, return a 'REF_TRANSACTION_ERROR_CREATE_EXISTS' error. This
allows batched updates to reject the individual update which conflicts
with the existing file, while updating the rest of the references.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>