git-gui: Handle starting on mapped shares under Cygwin

git-gui: Handle starting on mapped shares under Cygwin

I really cannot explain Cygwin's behavior here but if we start
git-gui through Cygwin on a local drive it appears that Cygwin
is leaving $env(PATH) in Unix style, even if it started a native
(non-Cygwin) Tcl/Tk process to run git-gui.  Yet starting that
same git-gui and Tcl/Tk combination through Cygwin on a network
share causes it to automatically convert $env(PATH) into Windows
style, which broke our internal "which" implementation.

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