bisect view: check for MinGW32 and MacOSX in addition to X11

When deciding if gitk or git-log should be used to visualize the current
state, the environment variable DISPLAY was checked.  Now, we check
MSYSTEM (for MinGW32/MSys) and SECURITYSESSIONID (for MacOSX) in addition.

Note that there is currently no way to ssh into MinGW32, and that
SECURITYSESSIONID is not set automatically on MacOSX when ssh'ing into it.
So this patch should be safe.

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

git-bisect.sh

@@ -331,9 +331,9 @@ bisect_visualize() {
 
 	if test $# = 0
 	then
-		case "${DISPLAY+set}" in
+		case "${DISPLAY+set}${MSYSTEM+set}${SECURITYSESSIONID+set}" in
 		'')	set git log ;;
-		set)	set gitk ;;
+		set*)	set gitk ;;
 		esac
 	else
 		case "$1" in