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>

git-gui.sh

@@ -305,7 +305,7 @@ proc _which {what} {
 	global env _search_exe _search_path
 
 	if {$_search_path eq {}} {
-		if {[is_Cygwin]} {
+		if {[is_Cygwin] && [regexp {^(/|\.:)} $env(PATH)]} {
 			set _search_path [split [exec cygpath \
 				--windows \
 				--path \