git-gui: Created very crude Tools menu, to support miga.

In one particular case I have a tool called 'miga' which users may need
to invoke on their repository.  This is a homegrown tool which is not
(and should be) part of git-gui, but I still want to be able to run it
from within the gui.

Right now I'm taking a shortcut and adding it to the Tools menu if
we are not on Mac OS X and the support script used to launch the tool
exists in the local filesystem.  This is nothing but a complete and
utter hack.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>