storage: reuse deltas from packfiles

* plumbing: add DeltaObject interface for EncodedObjects that
  are deltas and hold additional information about them, such
  as the hash of the base object.

* plumbing/storer: add DeltaObjectStorer interface for object
  storers that can return DeltaObject. Note that calls to
  EncodedObject will never return instances of DeltaObject.
  That requires explicit calls to DeltaObject.

* storage/filesystem: implement DeltaObjectStorer interface.

* plumbing/packfile: packfile encoder now supports reusing
  deltas that are already computed (e.g. from an existing
  packfile) if the storage implements DeltaObjectStorer.
  Reusing deltas boosts performance of packfile generation
  (e.g. on push).

1 files changed, 8 insertions, 0 deletions

diff --git a/plumbing/storer/object.go b/plumbing/storer/object.go
index a733ee6..3f41468 100644
--- a/plumbing/storer/object.go
+++ b/plumbing/storer/object.go
@@ -38,6 +38,14 @@ type EncodedObjectStorer interface {
 	IterEncodedObjects(plumbing.ObjectType) (EncodedObjectIter, error)
 }
 
+// DeltaObjectStorer is an EncodedObjectStorer that can return delta
+// objects.
+type DeltaObjectStorer interface {
+	// DeltaObject is the same as EncodedObject but without resolving deltas.
+	// Deltas will be returned as plumbing.DeltaObject instances.
+	DeltaObject(plumbing.ObjectType, plumbing.Hash) (plumbing.EncodedObject, error)
+}
+
 // Transactioner is a optional method for ObjectStorer, it enable transaction
 // base write and read operations in the storage
 type Transactioner interface {