msvc: support building Git using MS Visual C++

With this patch, Git can be built using the Microsoft toolchain, via:

	make MSVC=1 [DEBUG=1]

Third party libraries are built from source using the open source
"vcpkg" tool set. See https://github.com/Microsoft/vcpkg

On a first build, the vcpkg tools and the third party libraries are
automatically downloaded and built. DLLs for the third party libraries
are copied to the top-level (and t/helper) directory to facilitate
debugging. See compat/vcbuild/README.

A series of .bat files are invoked by the Makefile to find the location
of the installed version of Visual Studio and the associated compiler
tools (essentially replicating the environment setup performed by a
"Developer Command Prompt"). This should find the most recent VS2015 or
VS2017 installation. Output from these scripts are used by the Makefile
to define compiler and linker pathnames and -I and -L arguments.

The build produces .pdb files for both debug and release builds.

Note: This commit was squashed from an organic series of commits
developed between 2016 and 2018 in Git for Windows' `master` branch.
This combined commit eliminates the obsolete commits related to fetching
NuGet packages for third party libraries. It is difficult to use NuGet
packages for C/C++ sources because they may be built by earlier versions
of the MSVC compiler and have CRT version and linking issues.

Additionally, the C/C++ NuGet packages that we were using tended to not
be updated concurrently with the sources.  And in the case of cURL and
OpenSSL, this could expose us to security issues.

Helped-by: Yue Lin Ho <b8732003@student.nsysu.edu.tw>
Helped-by: Philip Oakley <philipoakley@iee.org>
Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
