ci(visual-studio): use strict compile flags, and optimization

To make full use of the work that went into the Visual Studio build & test jobs in our CI/PR builds, let's turn on strict compiler flags. This will give us the benefit of Visual C's compiler warnings (which, at times, seem to catch things that GCC does not catch, and vice versa). While at it, also turn on optimization; It does not make sense to produce binaries with debug information, and we can use any ounce of speed that we get (because the test suite is particularly slow on Windows, thanks to the need to run inside a Unix shell, which requires us to use the POSIX emulation layer provided by MSYS2). Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>