refs_ref_iterator_begin(): don't check prefixes redundantly

refs_ref_iterator_begin(): don't check prefixes redundantly

The backend already correctly restricts its output to references whose
names start with the prefix. By passing the prefix again to
`prefix_ref_iterator`, we were forcing that iterator to do redundant
prefix comparisons. So set it to the empty string.

Signed-off-by: Michael Haggerty <redacted>
Signed-off-by: Junio C Hamano <redacted>