refs.c: allow multiple reflog updates during a single transaction

refs.c: allow multiple reflog updates during a single transaction

Allow to make multiple reflog updates to the same ref during a transaction.
This means we only need to lock the reflog once, during the first update
that touches the reflog, and that all further updates can just write the
reflog entry since the reflog is already locked.

This allows us to write code such as:

t = transaction_begin()
transaction_reflog_update(t, "foo", REFLOG_TRUNCATE, NULL);
loop-over-somehting...
   transaction_reflog_update(t, "foo", 0, <message>);
transaction_commit(t)

where we first truncate the reflog and then build the new content one line at a
time.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>