Improve write_archive performance

[xMez]

Accessing each item in the tree during this loop slows down the write_archive method significantly when dealing with repositories with a large nr of files.

On a repository of ~5000 files the archiving time went from 13s to 0.5s.
From what I can tell when testing tree[entry.path].filemode and entry.mode are the same.

[jdavid]

Thanks