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.

2 files changed, 13 insertions, 11 deletions

diff --git a/examples/showcase/main.go b/examples/showcase/main.go
index ff5c7d3..4d1206d 100644
--- a/examples/showcase/main.go
+++ b/examples/showcase/main.go
@@ -6,6 +6,8 @@ import (
 	"strings"
 
 	"gopkg.in/src-d/go-git.v4"
+	"gopkg.in/src-d/go-git.v4/plumbing/object"
+
 	. "gopkg.in/src-d/go-git.v4/examples"
 )
 
@@ -42,7 +44,7 @@ func main() {
 	CheckIfError(err)
 
 	// ... get the files iterator and print the file
-	tree.Files().ForEach(func(f *git.File) error {
+	tree.Files().ForEach(func(f *object.File) error {
 		fmt.Printf("100644 blob %s    %s\n", f.Hash, f.Name)
 		return nil
 	})
diff --git a/examples/storage/aerospike/storage.go b/examples/storage/aerospike/storage.go
index a1d8c00..87d4a29 100644
--- a/examples/storage/aerospike/storage.go
+++ b/examples/storage/aerospike/storage.go
@@ -33,11 +33,11 @@ func NewStorage(client *driver.Client, ns, url string) (*Storage, error) {
 	return &Storage{client: client, ns: ns, url: url}, nil
 }
 
-func (s *Storage) NewObject() plumbing.Object {
+func (s *Storage) NewEncodedObject() plumbing.EncodedObject {
 	return &plumbing.MemoryObject{}
 }
 
-func (s *Storage) SetObject(obj plumbing.Object) (plumbing.Hash, error) {
+func (s *Storage) SetEncodedObject(obj plumbing.EncodedObject) (plumbing.Hash, error) {
 	key, err := s.buildKey(obj.Hash(), obj.Type())
 	if err != nil {
 		return obj.Hash(), err
@@ -64,7 +64,7 @@ func (s *Storage) SetObject(obj plumbing.Object) (plumbing.Hash, error) {
 	return obj.Hash(), err
 }
 
-func (s *Storage) Object(t plumbing.ObjectType, h plumbing.Hash) (plumbing.Object, error) {
+func (s *Storage) EncodedObject(t plumbing.ObjectType, h plumbing.Hash) (plumbing.EncodedObject, error) {
 	key, err := s.buildKey(h, t)
 	if err != nil {
 		return nil, err
@@ -82,7 +82,7 @@ func (s *Storage) Object(t plumbing.ObjectType, h plumbing.Hash) (plumbing.Objec
 	return objectFromRecord(rec, t)
 }
 
-func (s *Storage) IterObjects(t plumbing.ObjectType) (storer.ObjectIter, error) {
+func (s *Storage) IterEncodedObjects(t plumbing.ObjectType) (storer.EncodedObjectIter, error) {
 	stmnt := driver.NewStatement(s.ns, t.String())
 	err := stmnt.Addfilter(driver.NewEqualFilter(urlField, s.url))
 
@@ -91,19 +91,19 @@ func (s *Storage) IterObjects(t plumbing.ObjectType) (storer.ObjectIter, error)
 		return nil, err
 	}
 
-	return &ObjectIter{t, rs.Records}, nil
+	return &EncodedObjectIter{t, rs.Records}, nil
 }
 
 func (s *Storage) buildKey(h plumbing.Hash, t plumbing.ObjectType) (*driver.Key, error) {
 	return driver.NewKey(s.ns, t.String(), fmt.Sprintf("%s|%s", s.url, h.String()))
 }
 
-type ObjectIter struct {
+type EncodedObjectIter struct {
 	t  plumbing.ObjectType
 	ch chan *driver.Record
 }
 
-func (i *ObjectIter) Next() (plumbing.Object, error) {
+func (i *EncodedObjectIter) Next() (plumbing.EncodedObject, error) {
 	r := <-i.ch
 	if r == nil {
 		return nil, io.EOF
@@ -112,7 +112,7 @@ func (i *ObjectIter) Next() (plumbing.Object, error) {
 	return objectFromRecord(r, i.t)
 }
 
-func (i *ObjectIter) ForEach(cb func(obj plumbing.Object) error) error {
+func (i *EncodedObjectIter) ForEach(cb func(obj plumbing.EncodedObject) error) error {
 	for {
 		obj, err := i.Next()
 		if err != nil {
@@ -133,9 +133,9 @@ func (i *ObjectIter) ForEach(cb func(obj plumbing.Object) error) error {
 	}
 }
 
-func (i *ObjectIter) Close() {}
+func (i *EncodedObjectIter) Close() {}
 
-func objectFromRecord(r *driver.Record, t plumbing.ObjectType) (plumbing.Object, error) {
+func objectFromRecord(r *driver.Record, t plumbing.ObjectType) (plumbing.EncodedObject, error) {
 	content := r.Bins["blob"].([]byte)
 
 	o := &plumbing.MemoryObject{}