win32: add a helper to run git.exe without a foreground window

On Windows, there are two kinds of executables, console ones and
non-console ones. Git's executables are all console ones.

When launching the former e.g. in a scheduled task, a CMD window pops
up. This is not what we want for the tasks installed via the `git
maintenance` command.

To work around this, let's introduce `headless-git.exe`, which is a
non-console program that does _not_ pop up any window. All it does is to
re-launch `git.exe`, suppressing that console window, passing through
all command-line arguments as-are.

Helped-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Helped-by: Yuyi Wang <Strawberry_Str@hotmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.mak.uname

@@ -526,6 +526,8 @@ else
 endif
 	X = .exe
 
+	EXTRA_PROGRAMS += headless-git$X
+
 compat/msvc.o: compat/msvc.c compat/mingw.c GIT-CFLAGS
 endif
 ifeq ($(uname_S),Interix)
@@ -705,6 +707,7 @@ ifeq ($(uname_S),MINGW)
 	COMPAT_CFLAGS += -D__USE_MINGW_ANSI_STDIO=0 -DDETECT_MSYS_TTY \
 		-fstack-protector-strong
 	EXTLIBS += -lntdll
+	EXTRA_PROGRAMS += headless-git$X
 	INSTALL = /bin/install
 	INTERNAL_QSORT = YesPlease
 	HAVE_LIBCHARSET_H = YesPlease