A slight inconvenience with 'git archive --format=tar'

I just stumbled upon this while checking a few mailing lists.
I haven't found any mails about in in the archives yet, so I assume, that
no mail have been written yet.

The problem is described here:
http://sourceforge.net/projects/sevenzip/forums/forum/45798/topic/5322604

Basically, while this is not a problem for GNU tar, the correct checksum
should be computed using unsigned values.

Attached trivial testcase shows the difference.

Patch making the change shown in the testcase also attached.