Make an explicit bitcoin_txid() call.

Neater than open-coding it.  We still need the raw version for
signatures though.

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

anchor.c

@@ -97,7 +97,6 @@ void anchor_txid(struct bitcoin_tx *anchor,
 	Pkt *p1, *p2;
 	LeakAnchorSigsAndPretendWeDidnt *leak1, *leak2;
 	size_t i;
-	struct sha256_ctx shactx;
 	
 	p1 = pkt_from_file(leakfile1, PKT__PKT_OMG_FAIL);
 	p2 = pkt_from_file(leakfile2, PKT__PKT_OMG_FAIL);
@@ -121,9 +120,7 @@ void anchor_txid(struct bitcoin_tx *anchor,
 			= leak2->sigs->script[i].len;
 	}
 
-	sha256_init(&shactx);
-	sha256_tx(&shactx, anchor);
-	sha256_double_done(&shactx, txid);
+	bitcoin_txid(anchor, txid);
 
 	pkt__free_unpacked(p1, NULL);
 	pkt__free_unpacked(p2, NULL);

bitcoin_tx.c

@@ -64,6 +64,14 @@ void sha256_tx(struct sha256_ctx *ctx, const struct bitcoin_tx *tx)
 	sha256_le32(ctx, tx->lock_time);
 }
 
+void bitcoin_txid(const struct bitcoin_tx *tx, struct sha256_double *txid)
+{
+	struct sha256_ctx ctx = SHA256_INIT;
+
+	sha256_tx(&ctx, tx);
+	sha256_double_done(&ctx, txid);
+}
+
 struct bitcoin_tx *bitcoin_tx(const tal_t *ctx, varint_t input_count,
 			      varint_t output_count)
 {

bitcoin_tx.h

@@ -32,9 +32,12 @@ struct bitcoin_tx_input {
 	u32 sequence_number;
 };
 
-/* Linearize the tx.  This is good for deriving the txid, as well as the
- * signature. */
-void sha256_tx(struct sha256_ctx *shactx, const struct bitcoin_tx *tx);
+
+/* SHA256^2 the tx: simpler than sha256_tx */
+void bitcoin_txid(const struct bitcoin_tx *tx, struct sha256_double *txid);
+
+/* Useful for signature code. */
+void sha256_tx(struct sha256_ctx *ctx, const struct bitcoin_tx *tx);
 
 /* Allocate a tx: you just need to fill in inputs and outputs (they're
  * zeroed with inputs' sequence_number set to FFFFFFFF) */