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, 11 insertions, 0 deletions

diff --git a/plumbing/object.go b/plumbing/object.go
index 3304da2..63396f0 100644
--- a/plumbing/object.go
+++ b/plumbing/object.go
@@ -23,6 +23,17 @@ type EncodedObject interface {
 	Writer() (io.WriteCloser, error)
 }
 
+// DeltaObject is an EncodedObject representing a delta.
+type DeltaObject interface {
+	EncodedObject
+	// BaseHash returns the hash of the object used as base for this delta.
+	BaseHash() Hash
+	// ActualHash returns the hash of the object after applying the delta.
+	ActualHash() Hash
+	// Size returns the size of the object after applying the delta.
+	ActualSize() int64
+}
+
 // ObjectType internal object type
 // Integer values from 0 to 7 map to those exposed by git.
 // AnyObject is used to represent any from 0 to 7.