Update buildah to current master

Vendor some changes to parsing code that we need for Podman.

Signed-off-by: Matthew Heon <matthew.heon@pm.me>

vendor/github.com/containers/buildah/digester.go

@@ -0,0 +1,64 @@
+package buildah
+
+import (
+	"hash"
+	"strings"
+
+	digest "github.com/opencontainers/go-digest"
+)
+
+type singleDigester struct {
+	digester digest.Digester
+	prefix   string
+}
+
+// CompositeDigester can compute a digest over multiple items.
+type CompositeDigester struct {
+	digesters []singleDigester
+}
+
+// Restart clears all state, so that the composite digester can start over.
+func (c *CompositeDigester) Restart() {
+	c.digesters = nil
+}
+
+// Start starts recording the digest for a new item.  The caller should call
+// Hash() immediately after to retrieve the new io.Writer.
+func (c *CompositeDigester) Start(prefix string) {
+	prefix = strings.TrimSuffix(prefix, ":")
+	c.digesters = append(c.digesters, singleDigester{digester: digest.Canonical.Digester(), prefix: prefix})
+}
+
+// Hash returns the hasher for the current item.
+func (c *CompositeDigester) Hash() hash.Hash {
+	num := len(c.digesters)
+	if num == 0 {
+		return nil
+	}
+	return c.digesters[num-1].digester.Hash()
+}
+
+// Digest returns the prefix and a composite digest over everything that's been
+// digested.
+func (c *CompositeDigester) Digest() (string, digest.Digest) {
+	num := len(c.digesters)
+	switch num {
+	case 0:
+		return "", ""
+	case 1:
+		return c.digesters[0].prefix, c.digesters[0].digester.Digest()
+	default:
+		content := ""
+		for i, digester := range c.digesters {
+			if i > 0 {
+				content += ","
+			}
+			prefix := digester.prefix
+			if digester.prefix != "" {
+				digester.prefix += ":"
+			}
+			content += prefix + digester.digester.Digest().Encoded()
+		}
+		return "multi", digest.Canonical.FromString(content)
+	}
+}