builtin/bundle: have unbundle check for repo before opening its bundle

builtin/bundle: have unbundle check for repo before opening its bundle

The `git bundle unbundle` subcommand requires a repository to unbundle
the contents into. As thus, the subcommand checks whether we have a
startup repository in the first place, and if not it dies.

This check happens after we have already opened the bundle though. This
causes a segfault when running outside of a repository starting with
c8aed5e8da (repository: stop setting SHA1 as the default object hash,
2024-05-07) because we have no hash function set up, but we do try to
parse refs advertised by the bundle's header.

The next commit will fix that underlying issue by defaulting to the SHA1
object format for bundles, which will also fix the described segfault here.
But as we know that we will die anyway, we can do better than that and
avoid some vain work by moving the check for a repository before we try
to open the bundle.

Reported-by: ArcticLampyrid <ArcticLampyrid@outlook.com>
Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>