msvc: add a Makefile target to pre-generate the Visual Studio solution

The entire idea of generating the VS solution makes only sense if we
generate it via Continuous Integration; otherwise potential users would
still have to download the entire Git for Windows SDK.

If we pre-generate the Visual Studio solution, Git can be built entirely
within Visual Studio, and the test scripts can be run in a regular Git
for Windows (e.g. the Portable Git flavor, which does not include a full
GCC toolchain and therefore weighs only about a tenth of Git for
Windows' SDK).

So let's just add a target in the Makefile that can be used to generate
said solution; The generated files will then be committed so that they
can be pushed to a branch ready to check out by Visual Studio users.

To make things even more useful, we also generate and commit other files
that are required to run the test suite, such as templates and
bin-wrappers: with this, developers can run the test suite in a regular
Git Bash after building the solution in Visual Studio.

Note: for this build target, we do not actually need to initialize the
`vcpkg` system, so we don't.

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