refs: extract out reflog config to generic layer

refs: extract out reflog config to generic layer

The reference backends need to know when to create reflog entries, this
is dictated by the 'core.logallrefupdates' config. Instead of relying on
the backends to call `repo_settings_get_log_all_ref_updates()` to obtain
this config value, let's do this in the generic layer and pass down the
value to the backends.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>