In preparation for introducing an abstraction around a collection of
refspecs (much like how a 'struct pathspec' is a collection of 'struct
pathspec_item's) rename the existing 'struct refspec' to 'struct
refspec_item'.
Signed-off-by: Brandon Williams <bmwill@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>