git merge: reword failure message.

99.9999% of the time, the command is used with a single
strategy; after a merge failure, saying "No strategy handled the
merge" is technically correct, but there is no point stressing
we tried and failed all the possibilities the user has given.

Just say that it failed.

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

git-merge.sh

@@ -400,7 +400,14 @@ fi
 case "$best_strategy" in
 '')
 	restorestate
+	case "$use_strategies" in
+	?*' '?*)
 		echo >&2 "No merge strategy handled the merge."
+		;;
+	*)
+		echo >&2 "Merge with strategy $use_strategies failed."
+		;;
+	esac
 	exit 2
 	;;
 "$wt_strategy")