bundle: support fsck message configuration

bundle: support fsck message configuration

If the `VERIFY_BUNDLE_FLAG` is set during `unbundle()`, the
git-index-pack(1) spawned is configured with the `--fsck-options` flag
to perform fsck verification. With this flag enabled, there is not a way
to configure fsck message severity though.

Extend the `unbundle_opts` type to store fsck message severity
configuration and update `unbundle()` to conditionally append it to the
`--fsck-objects` flag if provided. This enables `unbundle()` call sites
to support optionally setting the severity for specific fsck messages.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>