builtin/clone: fix bundle URIs with mismatching object formats

builtin/clone: fix bundle URIs with mismatching object formats

We create the reference database in git-clone(1) quite early before
connecting to the remote repository. Given that we do not yet know about
the object format that the remote repository uses at that point in time
the consequence is that the refdb may be initialized with the wrong
object format.

This is not a problem in the context of the files backend as we do not
encode the object format anywhere, and furthermore the only reference
that we write between initializing the refdb and learning about the
object format is the "HEAD" symref. It will become a problem though once
we land the reftable backend, which indeed does require to know about
the proper object format at the time of creation. We thus need to
rearrange the logic in git-clone(1) so that we only initialize the refdb
once we have learned about the actual object format.

As a first step, move listing of remote references to happen earlier,
which also allow us to set up the hash algorithm of the repository
earlier now. While we aim to execute this logic as late as possible
until after most of the setup has happened already, detection of the
object format and thus later the setup of the reference database must
happen before any other logic that may spawn Git commands or otherwise
these Git commands may not recognize the repository as such.

The first Git step where we expect the repository to be fully initalized
is when we fetch bundles via bundle URIs. Funny enough, the comments
there also state that "the_repository must match the cloned repo", which
is indeed not necessarily the case for the hash algorithm right now. So
in practice it is the right thing to detect the remote's object format
before downloading bundle URIs anyway, and not doing so causes clones
with bundle URIs to fail when the local default object format does not
match the remote repository's format.

Unfortunately though, this creates a new issue: downloading bundles may
take a long time, so if we list refs beforehand they might've grown
stale meanwhile. It is not clear how to solve this issue except for a
second reference listing though after we have downloaded the bundles,
which may be an expensive thing to do.

Arguably though, it's preferable to have a staleness issue compared to
being unable to clone a repository altogether.

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