make 'gotadmin load' always read data from standard input 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@

add an initial implementation of gotadmin load 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