fast-export: add new --refspec option

So that we can convert the exported ref names.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
transport-helper: add support for old:new refspec

By using fast-export's new --refspec option.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
fast-import: add support to delete refs

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
fast-export: add support to delete refs

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
transport-helper: add support to delete branches

For remote-helpers that use 'export' to push.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
transport-helper: don't update refs in dry-run

The remote helper namespace should not be updated.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>