Factor out a small logic out of the private write_pack_file() function
in builtin/pack-objects.c.

This changes the order of finishing multi-pack generation slightly. The
code used to

 - adjust shared perm of temporary packfile
 - rename temporary packfile to the final name
 - update mtime of the packfile under the final name
 - adjust shared perm of temporary idxfile
 - rename temporary idxfile to the final name

but because the helper does not want to do the mtime thing, the updated
code does that step first and then all the rest.

Signed-off-by: Junio C Hamano <gitster@pobox.com>