fixup! git-gui: fix detection of Cygwin

Author: dscho

git-gui/git-gui.sh

@@ -270,10 +270,12 @@ proc is_Windows {} {
 proc is_Cygwin {} {
 	global _iscygwin
 	if {$_iscygwin eq {}} {
-		if {$::tcl_platform(platform) eq {windows} &&
-				(![info exists ::env(MSYSTEM)] ||
-				 $::env(MSYSTEM) eq {MSYS})} {
-			set _iscygwin 1
+		if {$::tcl_platform(platform) eq {windows}} {
+			if {[catch {set p [exec cygpath --windir]} err]} {
+				set _iscygwin 0
+			} else {
+				set _iscygwin 1
+			}
 		} else {
 			set _iscygwin 0
 		}