contrib/short_channel_id-to-txid.sh: simple mapping util.

I use this to look up on smartbit.com.au to see if a channel is spent,
for example.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

contrib/short_channel_id-to-txid.sh

@@ -0,0 +1,15 @@
+#! /bin/sh
+
+set -e
+
+if [ "$#" != 1 ]; then
+    echo Usage: "$0" "short:channel:id" >&2
+    echo Uses bitcoin-cli to extract the actual txid >&2
+    exit 1
+fi
+
+BLOCK=$(echo "$1" | cut -d: -f1)
+TXNUM=$(echo "$1" | cut -d: -f2)
+
+bitcoin-cli getblock "$(bitcoin-cli getblockhash "$BLOCK")" true | grep '^    "' | head -n "$((TXNUM + 1))" | tail -n 1 | tr -dc '0-9a-f\n'
+