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>

contrib/buildsystems/engine.pl

@@ -371,6 +371,7 @@ sub handleLinkLine
 #    exit(1);
     foreach (@objfiles) {
         my $sourcefile = $_;
+        $sourcefile =~ s/^headless-git\.o$/compat\/win32\/headless.c/;
         $sourcefile =~ s/\.o$/.c/;
         push(@sources, $sourcefile);
         push(@cflags, @{$compile_options{"${sourcefile}_CFLAGS"}});