refs.c: make ref_update_reject_duplicates take a strbuf argument for errors

refs.c: make ref_update_reject_duplicates take a strbuf argument for errors

Make ref_update_reject_duplicates return any error that occurs through a
new strbuf argument. This means that when a transaction commit fails in
this function we will now be able to pass a helpful error message back to the
caller.

Reviewed-by: Jonathan Nieder <redacted>
Signed-off-by: Ronnie Sahlberg <redacted>
Signed-off-by: Junio C Hamano <redacted>
Acked-by: Michael Haggerty <redacted>