Move chmod(foo, 0444) into move_temp_to_file()

Move chmod(foo, 0444) into move_temp_to_file()

When writing out a loose object or a pack (index), move_temp_to_file() is
called to finalize the resulting file. These files (loose files and packs)
should all have permission mode 0444 (modulo adjust_shared_perm()).
Therefore, instead of doing chmod(foo, 0444) explicitly from each callsite
(or even forgetting to chmod() at all), do the chmod() call from within
move_temp_to_file().

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>