make alloc_ref_from_str() the new alloc_ref()

make alloc_ref_from_str() the new alloc_ref()

With all calls to alloc_ref() gone, we can remove it and then we're free
to give alloc_ref_from_str() the shorter name.  It's a much nicer
interface, as the callers always need to have a name string when they
allocate a ref anyway and don't need to calculate and pass its length+1
any more.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>