bisect view: call gitk if Cygwin's SESSIONNAME variable is set

It seems that Cygwin sets the variable SESSIONNAME when an interactive
desktop session is running, and does not set it when you log in via ssh.

So we can use this variable to determine whether to run gitk or git log
in git bisect view.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-bisect.sh

@@ -508,7 +508,7 @@ bisect_visualize() {
 
 	if test $# = 0
 	then
-		case "${DISPLAY+set}${MSYSTEM+set}${SECURITYSESSIONID+set}" in
+		case "${DISPLAY+set}${SESSIONNAME+set}${MSYSTEM+set}${SECURITYSESSIONID+set}" in
 		'')	set git log ;;
 		set*)	set gitk ;;
 		esac