git/odb/pack: introduce *Chain type

git/odb/pack/chain.go

@@ -0,0 +1,21 @@
+package pack
+
+// Chain represents an element in the delta-base chain corresponding to a packed
+// object.
+type Chain interface {
+	// Unpack unpacks the data encoded in the delta-base chain up to and
+	// including the receiving Chain implementation by applying the
+	// delta-base chain successively to itself.
+	//
+	// If there was an error in the delta-base resolution, i.e., the chain
+	// is malformed, has a bad instruction, or there was an mmap-related
+	// read error, this function is expected to return that error.
+	//
+	// In the event that a non-nil error is returned, it is assumed that the
+	// unpacked data this function returns is malformed, or otherwise
+	// corrupt.
+	Unpack() ([]byte, error)
+
+	// Type returns the type of the receiving chain element.
+	Type() PackedObjectType
+}

git/odb/pack/chain_test.go

@@ -0,0 +1,12 @@
+package pack
+
+type ChainSimple struct {
+	X   []byte
+	Err error
+}
+
+func (c *ChainSimple) Unpack() ([]byte, error) {
+	return c.X, c.Err
+}
+
+func (c *ChainSimple) Type() PackedObjectType { return TypeNone }