transport-helper: add 'force' to 'export' helpers

Otherwise they cannot know when to force the push or not (other than
hacks).

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
transport-helper: check for 'forced update' message

So the remote-helpers can tell us when a forced push was needed.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
fast-export: improve argument parsing

We don't want to pass arguments specific to fast-export to
setup_revisions.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
fast-export: add new --refspec option

So that we can covert 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>