Vcproj.pm: auto-generate GUIDs

Vcproj.pm: auto-generate GUIDs

We ran out GUIDs. Again. But there is no need to: we can generate them
semi-randomly from the target file name of the project.

Note: the Vcproj generator is probably only interesting for historical
reasons; nevertheless, the upcoming Vcxproj generator (to support modern
Visual Studio versions) is based on the Vcproj generator and it is
better to fix this here first.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>