windows: include the Python bits when building Git for Windows

windows: include the Python bits when building Git for Windows

While Git for Windows does not _ship_ Python (in order to save on
bandwidth), MSYS2 provides very fine Python interpreters that users can
easily take advantage of, by using Git for Windows within its SDK.

Previously, we excluded the Python bits, mostly due to historical
reasons: In the Git for Windows v1.x days, we built Git using
MSys/MinGW, without support for any Python scripts.

Therefore, let's move out the `NO_PYTHON` definition from the generic
part of the MINGW section (which includes special handling for MSYS2/Git
for Windows, for the long-superseded msysGit environment, as well as for
the setup of probably just one developer remaining with their MSys1)
into the two sections that cover different environments than Git for
Windows' SDK.

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