move plumbing from top level package to plumbing (#183)

* plumbing: rename Object -> EncodedObject.

* plumbing/storer: rename ObjectStorer -> EncodedObjectStorer.

* move difftree to plumbing/difftree.

* move diff -> utils/diff

* make Object/Tag/Blob/Tree/Commit/File depend on storer.

* Object and its implementations now depend only on
  storer.EncodedObjectStorer, not git.Repository.
* Tests are decoupled accordingly.

* move Object/Commit/File/Tag/Tree to plumbing/object.

* move Object/Commit/File/Tag/Tree to plumbing/object.
* move checkClose to utils/ioutil.

* move RevListObjects to plumbing/revlist.Objects.

* move DiffTree to plumbing/difftree package.

* rename files with plural nouns to singular

* plumbing/object: add GetBlob/GetCommit/GetTag/GetTree.

1 files changed, 6 insertions, 5 deletions

diff --git a/plumbing/format/packfile/object_pack.go b/plumbing/format/packfile/object_pack.go
index dc5a3c7..dfe9bb2 100644
--- a/plumbing/format/packfile/object_pack.go
+++ b/plumbing/format/packfile/object_pack.go
@@ -6,20 +6,21 @@ import "gopkg.in/src-d/go-git.v4/plumbing"
 // pack file.
 type ObjectToPack struct {
 	// The main object to pack, it could be any object, including deltas
-	Object plumbing.Object
+	Object plumbing.EncodedObject
 	// Base is the object that a delta is based on (it could be also another delta).
 	// If the main object is not a delta, Base will be null
 	Base *ObjectToPack
 	// Original is the object that we can generate applying the delta to
-	// Base, or the same object as Object in the case of a non-delta object.
-	Original plumbing.Object
+	// Base, or the same object as EncodedObject in the case of a non-delta
+	// object.
+	Original plumbing.EncodedObject
 	// Depth is the amount of deltas needed to resolve to obtain Original
 	// (delta based on delta based on ...)
 	Depth int
 }
 
 // newObjectToPack creates a correct ObjectToPack based on a non-delta object
-func newObjectToPack(o plumbing.Object) *ObjectToPack {
+func newObjectToPack(o plumbing.EncodedObject) *ObjectToPack {
 	return &ObjectToPack{
 		Object:   o,
 		Original: o,
@@ -29,7 +30,7 @@ func newObjectToPack(o plumbing.Object) *ObjectToPack {
 // newDeltaObjectToPack creates a correct ObjectToPack for a delta object, based on
 // his base (could be another delta), the delta target (in this case called original),
 // and the delta Object itself
-func newDeltaObjectToPack(base *ObjectToPack, original, delta plumbing.Object) *ObjectToPack {
+func newDeltaObjectToPack(base *ObjectToPack, original, delta plumbing.EncodedObject) *ObjectToPack {
 	return &ObjectToPack{
 		Object:   delta,
 		Base:     base,