git-merge: Exit with code 2 if no strategy was able to handle the merge.

This way it is possible to test in scripts if the merge was non-clean
or if the strategy had other problems with the merge.

Signed-off-by: Fredrik Kuivinen <freku045@student.liu.se>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-merge.sh

@@ -273,7 +273,8 @@ fi
 case "$best_strategy" in
 '')
 	restorestate
-	die "No merge strategy handled the merge."
+	echo >&2 "No merge strategy handled the merge."
+	exit 2
 	;;
 "$wt_strategy")
 	# We already have its result in the working tree.