This matches the existing use of "got -r path cmd" and "git_commit path args".

This provides better symmetry with 'gotadmin dump', and allows us to pass the list of references as command line arguments, replacing the -b option. The -l option now takes an argument which specifies the bundle file rather than requiring data to be fed on stdin with -l which feels awkward. ok op@

it is intended to be the counterpart of `gotadmin dump' and, just like it, there's planned support for handling fast-import stream. At the moment it only deals with git bundles. ok stsp