git-gui: use /cmd/git-gui.exe for shortcut

git-gui: use /cmd/git-gui.exe for shortcut

git-gui on Windows creates a shortcut that presumes the git-gui script
will run on the basic Windows environment as configured. But, Git for
Windows uses wrapper scripts to launch executables, assuring the
environment is correct (see [1] for details). The launcher for git-gui
is /cmd/git-gui.exe, is not on PATH, and is not detected or used by the
current shortcut code. Let's look for this before trying the existing
approaches.

[1] https://gitforwindows.org/git-wrapper.html

Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>