Implement credential commit object, but don't use it anywhere yet

---
type: change
message: Implement credential commit object, but don't use it anywhere yet
change_hash: AIJRxhlQQuDhoByl1nApcFrRhlS9bK+4w/6JYl8SHl9o
credentials:
- type: pgp_signature
  pub_key_id: 95C46FA6A41148AC
  body: 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
  account: mediocregopher

SPEC.md

@@ -193,10 +193,10 @@ credentials:
 
 ## Credential Commits
 
-Commits of type `credential` contain one or more credentials for some set of
-changes, and the change hash to which those credentials apply. The commit
-message head is not spec'd, but should be a human-readable description of "who
-is crediting what, and how".
+Commits of type `credential` contain one or more credentials for some hash
+(presumably a change hash, but in the future there may be other types).  The
+commit message head is not spec'd, but should be a human-readable description of
+"who is crediting what, and how".
 
 Example credential commit message:
 
@@ -204,7 +204,7 @@ Example credential commit message:
 some_user_id pgp sig of commits AAA..BBB with key CCC
 
 ---
-change_hash: XXX
+credentialed_hash: XXX
 credentials:
     - type: pgp_signature
       account_id: some_user_id

commit.go

@@ -40,6 +40,7 @@ type CommitInterface interface {
 // field should be set on a Commit, unless otherwise noted.
 type Commit struct {
 	Change     *CommitChange     `type:"change,default"`
+	Credential *CommitCredential `type:"credential"`
 
 	// Credentials represent all created Credentials for this commit, and can be
 	// set on all Commit objects regardless of other fields being set.

commit_signature.go

@@ -0,0 +1,46 @@
+package dehub
+
+import (
+	"dehub/yamlutil"
+	"encoding/base64"
+	"fmt"
+
+	"gopkg.in/src-d/go-git.v4/plumbing/object"
+)
+
+// CommitCredential describes the structure of a credential commit message.
+type CommitCredential struct {
+	CredentialedHash yamlutil.Blob `yaml:"credentialed_hash"`
+}
+
+var _ CommitInterface = CommitCredential{}
+
+// NewCommitCredential constructs and returns a Commit populated with a
+// CommitCredential encompassing the given hash. The Credentials of the returned
+// Commit will _not_ be filled in.
+func (r *Repo) NewCommitCredential(hash []byte) (Commit, error) {
+	return Commit{
+		Credential: &CommitCredential{
+			CredentialedHash: hash,
+		},
+	}, nil
+}
+
+// MessageHead implements the method for the CommitInterface interface.
+func (cc CommitCredential) MessageHead() (string, error) {
+	hash64 := base64.StdEncoding.EncodeToString(cc.CredentialedHash)
+	if len(hash64) > 6 {
+		hash64 = hash64[:6] + "..."
+	}
+	return fmt.Sprintf("Credential of hash %s", hash64), nil
+}
+
+// Hash implements the method for the CommitInterface.
+func (cc CommitCredential) Hash(_, _ *object.Tree) ([]byte, error) {
+	return cc.CredentialedHash, nil
+}
+
+// GetHash implements the method for the CommitInterface.
+func (cc CommitCredential) GetHash() []byte {
+	return cc.CredentialedHash
+}