ref-filter: introduce format_ref_array_item()

ref-filter: introduce format_ref_array_item()

To allow column display, we will need to first render the output in a
string list to allow print_columns() to compute the proper size of
each column before starting the actual output. Introduce the function
format_ref_array_item() that does the formatting of a ref_array_item
to an strbuf.

show_ref_array_item() is kept as a convenience wrapper around it which
obtains the strbuf and prints it the standard output.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Matthieu Moy <matthieu.moy@grenoble-inp.fr>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>