prune-packed: use strbuf to avoid having to worry about PATH_MAX

A/very/long/path/to/.git that becomes exactly PATH_MAX bytes long
after suffixed with /objects/??/??38-hex??, would have overflown
the on-stack pathname[] buffer.

Noticed-by:  Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>