import: use autodie, rely on PerlIO for retries

import: use autodie, rely on PerlIO for retries

As documented in perlipc(1), the default :perlio layer retries
the `read' perlop on EINTR.  The :perlio layer also makes `read'
perform read-in-full behavior; so there's no need to loop
ourselves.  Our responsibility is now only to detect short reads
in case fast-import is killed mid-stream.