Add git-zip-tree

Add git-zip-tree

In the Windows world ZIP files are better supported than tar files.
Windows even includes built-in support for ZIP files nowadays.

git-zip-tree is similar to git-tar-tree; it creates ZIP files out of
git trees.  It stores the commit ID (if available) in a ZIP file comment
which can be extracted by unzip.

There's still quite some room for improvement: this initial version
supports no symlinks, calls write() way too often (three times per file)
and there is no unit test.

[jc: with a minor typefix to avoid void* arithmetic]

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <junkio@cox.net>