update-ref --stdin: reimplement using reference transactions

update-ref --stdin: reimplement using reference transactions

This change is mostly clerical: the parse_cmd_*() functions need to
use local variables rather than a struct ref_update to collect the
arguments needed for each update, and then call ref_transaction_*() to
queue the change rather than building up the list of changes at the
caller side.

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