bisect: visualize with git-log if gitk is unavailable

If gitk is not available in the PATH, bisect ends up
exiting with the shell's 127 error code, confusing the git
wrapper into thinking that bisect is not a git command.

We already fallback to git-log if there doesn't seem to be a
graphical display available. We should do the same if gitk
is not available in our PATH at all. This not only fixes the
ugly error message, but is a much more sensible default than
failing to show the user anything.

Reported by Maxin John.

Tested-by: Maxin B. John <maxin@maxinbjohn.info>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-bisect.sh

@@ -288,10 +288,12 @@ bisect_visualize() {
 
 	if test $# = 0
 	then
-		case "${DISPLAY+set}${SESSIONNAME+set}${MSYSTEM+set}${SECURITYSESSIONID+set}" in
-		'')	set git log ;;
-		set*)	set gitk ;;
-		esac
+		if test -n "${DISPLAY+set}${SESSIONNAME+set}${MSYSTEM+set}${SECURITYSESSIONID+set}" &&
+		   type gitk >/dev/null 2>&1; then
+			set gitk
+		else
+			set git log
+		fi
 	else
 		case "$1" in
 		git*|tig) ;;