setup: unset ref storage when reinitializing repository version

setup: unset ref storage when reinitializing repository version

When reinitializing a repository's version we may end up unsetting the
hash algorithm when it matches the default hash algorithm. If we didn't
do that then the previously configured value might remain intact.

While the same issue exists for the ref storage extension, we don't do
this here. This has been fine for most of the part because it is not
supported to re-initialize a repository with a different ref storage
format anyway. We're about to introduce a new command to migrate ref
storages though, so this is about to become an issue there.

Prepare for this and unset the ref storage format when reinitializing a
repository with the "files" format.

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