define die() for scripts that use it.

As a fallout from not using git-sh-setup in scripts that can
operate from a subdirectory, we lost definition of die() from
them.  It might make sense to do some cleanup to consolidate
them back again, but this should suffice for now.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-merge-octopus.sh

@@ -8,6 +8,11 @@
 LF='
 '
 
+die () {
+    echo >&2 "$*"
+    exit 1
+}
+
 # The first parameters up to -- are merge bases; the rest are heads.
 bases= head= remotes= sep_seen=
 for arg