ref-filter: add function to print single ref_array_item
ref-filter functions are useful for printing git object information
using a format specifier. However, some other modules may not want to use
this functionality on a ref-array but only print a single item.
Expose a pretty_print_ref function to create, pretty print and free
individual ref-items.
Signed-off-by: Lukas Puehringer <luk.puehringer@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>