refs.c: rename log_ref_setup to create_reflog

Change the name from log_ref_setup to create_reflog. Make create_reflog
create the reflog unconditionally, whatever log_all_ref_updates is set to,
and instead check for log_all_ref_updates before calling it in log_ref_write.

Sicne create_reflog no longer checks log_all_ref_updates we can skip
temporarily setting it to true before calling create_reflog from
checkout.c.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>