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

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 <redacted>
Helped-by: Yuyi Wang <redacted>
Signed-off-by: Johannes Schindelin <redacted>
Signed-off-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>