vendor: update buildkit to master@31c870e82a48

Signed-off-by: Justin Chadwell <me@jedevc.com>

vendor/github.com/in-toto/in-toto-golang/in_toto/hashlib.go

@@ -0,0 +1,30 @@
+package in_toto
+
+import (
+	"crypto/sha256"
+	"crypto/sha512"
+	"hash"
+)
+
+/*
+getHashMapping returns a mapping from hash algorithm to supported hash
+interface.
+*/
+func getHashMapping() map[string]func() hash.Hash {
+	return map[string]func() hash.Hash{
+		"sha256": sha256.New,
+		"sha512": sha512.New,
+		"sha384": sha512.New384,
+	}
+}
+
+/*
+hashToHex calculates the hash over data based on hash algorithm h.
+*/
+func hashToHex(h hash.Hash, data []byte) []byte {
+	h.Write(data)
+	// We need to use h.Sum(nil) here, because otherwise hash.Sum() appends
+	// the hash to the passed data. So instead of having only the hash
+	// we would get: "dataHASH"
+	return h.Sum(nil)
+}