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 lonber checks log_all_ref_updates we can skip
temporarily setting it to true before calling create_reflog from
checkout.c.

log_ref_setup/create_reflog is relatively expensive, making many system
calls, and the common case will be that the reflog already exists.
Therefore use the much cheaper reflog_exists to check and only call
create_reflog if we actually do need to create the reflog when calling from
log_ref_write.

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