ref-filter: add function to print single ref_array_item

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>