Re: [PATCH 2/3] setup: fix reinit of repos with incompatible GIT_DEFAULT_REF_FORMAT

> The GIT_DEFAULT_REF_FORMAT environment variable can be set to influence
> the default ref format that new repostiories shall be initialized with.
> While this is the expected behaviour when creating a new repository, it
> is not when reinitializing a repository: we should retain the ref format
> currently used by it in that case.
>
> This doesn't work correctly right now:
>
>     $ git init --ref-format=files repo
>     Initialized empty Git repository in /tmp/repo/.git/
>     $ GIT_DEFAULT_REF_FORMAT=reftable git init repo
>     fatal: could not open '/tmp/repo/.git/refs/heads' for writing: Is a directory
>
> Instead of retaining the current ref format, the reinitialization tries
> to reinitialize the repository with the different format. This action
> fails when git-init(1) tries to write the ".git/refs/heads" stub, which
> in the context of the reftable backend is always written as a file so
> that we can detect clients which inadvertently try to access the repo
> with the wrong ref format. Seems like the protection mechanism works for
> this case, as well.