patchrefs: mark some symbols static

> Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
> ---
> 
> Hi David,
> 
> If you need to re-roll your 'dt/refs-backend-lmdb' branch, could
> you please squash the relevant parts of this patch into yours.
> 
> [yes, I didn't reference the movement of the external declaration
> in the commit message! :-D ]
> 
> Thanks!
> 
> ATB,
> Ramsay Jones
> 
>  refs.c               | 7 +++----
>  refs/files-backend.c | 2 +-
>  refs/refs-internal.h | 2 ++
>  3 files changed, 6 insertions(+), 5 deletions(-)
> 
> diff --git a/refs.c b/refs.c
> index 0be7065..c0faa97 100644
> --- a/refs.c
> +++ b/refs.c
> @@ -9,7 +9,7 @@
>  #include "object.h"
>  #include "tag.h"
>  
> -const char split_transaction_fail_warning[] =
> +static const char split_transaction_fail_warning[] =
>  	"A ref transaction was split across two refs backends.  Part of the "
>  	"transaction succeeded, but then the update to the per-worktree refs "
>  	"failed.  Your repository may be in an inconsistent state.";
> @@ -17,12 +17,11 @@ const char split_transaction_fail_warning[] =
>  /*
>   * We always have a files backend and it is the default.
>   */
> -extern struct ref_be refs_be_files;
> -struct ref_be *the_refs_backend = &refs_be_files;
> +static struct ref_be *the_refs_backend = &refs_be_files;
>  /*
>   * List of all available backends
>   */
> -struct ref_be *refs_backends = &refs_be_files;
> +static struct ref_be *refs_backends = &refs_be_files;
>  
>  const char *refs_backend_type;
>  
> diff --git a/refs/files-backend.c b/refs/files-backend.c
> index 0efc507..e8181ae 100644
> --- a/refs/files-backend.c
> +++ b/refs/files-backend.c
> @@ -3280,7 +3280,7 @@ static int ref_present(const char *refname,
>  	return string_list_has_string(affected_refnames, refname);
>  }
>  
> -void files_init_backend(void *data)
> +static void files_init_backend(void *data)
>  {
>  	/* do nothing */
>  }
> diff --git a/refs/refs-internal.h b/refs/refs-internal.h
> index 9c17fdf..8fb360d 100644
> --- a/refs/refs-internal.h
> +++ b/refs/refs-internal.h
> @@ -313,4 +313,6 @@ struct ref_be {
>  	for_each_replace_ref_fn *for_each_replace_ref;
>  };
>  
> +extern struct ref_be refs_be_files;
> +
>  #endif /* REFS_REFS_INTERNAL_H */