refs/files: lazy-load configuration to fix chicken-and-egg

refs/files: lazy-load configuration to fix chicken-and-egg

When initializing the "files" reference backend we read the repository's
config to parse "core.preferSymlinkRefs" and "core.logAllRefUpdates".
This results in a chicken-and-egg problem though, because parsing the
configuration may require us to have access to the reference store
already when an "onbranch" condition exists.

Luckily, all the configuration that we honor only relates to writing
references. Consequently, we don't strictly need that configuration to
be readily available at initialization time, and we can easiliy defer
parsing it to a later point in time.

Implement this fix and add tests that verify that we can indeed properly
parse these config knobs via an "onbranch" condition.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>