finish_tmp_packfile():use strbuf for pathname construction

finish_tmp_packfile():use strbuf for pathname construction

The old version fixes a maximum length on the buffer, which could be a problem
if one is not certain of the length of get_object_directory().
Using strbuf can avoid the protential bug.

Helped-by: Michael Haggerty <mhagger@alum.mit.edu>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Sun He <sunheehnus@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>