Implement an interoperability test for fetch-pack/upload-pack

Implement an interoperability test for fetch-pack/upload-pack

The next patches will extend the pack protocol. This test assures that this
extension is compatible to earlier versions of git-fetch-pack/git-upload-pack.

All you need to do to take advantage of this test, is to install older
known-to-be-working binaries in the path as "old-git-fetch-pack" and
"old-git-upload-pack".

Note that the warning when testing with old-git-fetch-pack is to be
expected (it just says that the old version was not taking advantage
of all the information which the server sent).

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>