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-add.sh

@@ -1,5 +1,10 @@
 #!/bin/sh
 
+die () {
+    echo >&2 "$*"
+    exit 1
+}
+
 usage() {
     die "usage: git add [-n] [-v] <file>..."
 }