builtin/prune.c: use strbuf to avoid having to worry about PATH_MAX

While at it, rename prune_tmp_object(), which used to be a helper to
remove temporary files that were created to become loose object
files, to prune_tmp_file(), as the function is also used to remove
any random cruft whose name begins with tmp_ directly in .git/object
or .git/object/pack directories these days.

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