fixup! git-gui: fix detection of Cygwin

The original fix unfortunately used the *inverted* logic: while we
wanted to say "this is Cygwin" when run in MSYS mode, we actually did
that when *not* running in MSYS mode.

This fixes git-for-windows#410

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

git-gui/git-gui.sh

@@ -272,7 +272,7 @@ proc is_Cygwin {} {
 	if {$_iscygwin eq {}} {
 		if {$::tcl_platform(platform) eq {windows} &&
 				(![info exists ::env(MSYSTEM)] ||
-				 $::env(MSYSTEM) ne {MSYS})} {
+				 $::env(MSYSTEM) eq {MSYS})} {
 			set _iscygwin 1
 		} else {
 			set _iscygwin 0