Add ability to peel any git object

Includes support for 'git_object_peel'.

object.go

@@ -22,6 +22,7 @@ type Object interface {
 	Id() *Oid
 	Type() ObjectType
 	Owner() *Repository
+	Peel(t ObjectType) (Object, error)
 }
 
 type gitObject struct {
@@ -69,6 +70,31 @@ func (o *gitObject) Free() {
 	C.git_object_free(o.ptr)
 }
 
+// Peel recursively peels an object until an object of the specified type is met.
+//
+// If the query cannot be satisfied due to the object model, ErrInvalidSpec
+// will be returned (e.g. trying to peel a blob to a tree).
+//
+// If you pass ObjectAny as the target type, then the object will be peeled
+// until the type changes. A tag will be peeled until the referenced object
+// is no longer a tag, and a commit will be peeled to a tree. Any other object
+// type will return ErrInvalidSpec.
+//
+// If peeling a tag we discover an object which cannot be peeled to the target
+// type due to the object model, an error will be returned.
+func (o *gitObject) Peel(t ObjectType) (Object, error) {
+	var cobj *C.git_object
+
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	if err := C.git_object_peel(&cobj, o.ptr, C.git_otype(t)); err < 0 {
+		return nil, MakeGitError(err)
+	}
+
+	return allocObject(cobj, o.repo), nil
+}
+
 func allocObject(cobj *C.git_object, repo *Repository) Object {
 	obj := gitObject{
 		ptr:  cobj,

object_test.go

@@ -102,3 +102,63 @@ func TestObjectOwner(t *testing.T) {
 	checkOwner(t, repo, commit)
 	checkOwner(t, repo, tree)
 }
+
+func TestObjectPeel(t *testing.T) {
+	repo := createTestRepo(t)
+	defer cleanupTestRepo(t, repo)
+
+	commitID, treeID := seedTestRepo(t, repo)
+
+	var obj Object
+
+	commit, err := repo.LookupCommit(commitID)
+	checkFatal(t, err)
+
+	obj, err = commit.Peel(ObjectAny)
+	checkFatal(t, err)
+
+	if obj.Type() != ObjectTree {
+		t.Fatalf("Wrong object type when peeling a commit, expected tree, have %v", obj.Type())
+	}
+
+	obj, err = commit.Peel(ObjectTag)
+
+	if !IsErrorCode(err, ErrInvalidSpec) {
+		t.Fatalf("Wrong error when peeling a commit to a tag, expected ErrInvalidSpec, have %v", err)
+	}
+
+	tree, err := repo.LookupTree(treeID)
+	checkFatal(t, err)
+
+	obj, err = tree.Peel(ObjectAny)
+
+	if !IsErrorCode(err, ErrInvalidSpec) {
+		t.Fatalf("Wrong error when peeling a tree, expected ErrInvalidSpec, have %v", err)
+	}
+
+	entry := tree.EntryByName("README")
+
+	blob, err := repo.LookupBlob(entry.Id)
+	checkFatal(t, err)
+
+	obj, err = blob.Peel(ObjectAny)
+
+	if !IsErrorCode(err, ErrInvalidSpec) {
+		t.Fatalf("Wrong error when peeling a blob, expected ErrInvalidSpec, have %v", err)
+	}
+
+	tagID := createTestTag(t, repo, commit)
+
+	tag, err := repo.LookupTag(tagID)
+	checkFatal(t, err)
+
+	obj, err = tag.Peel(ObjectAny)
+	checkFatal(t, err)
+
+	if obj.Type() != ObjectCommit {
+		t.Fatalf("Wrong object type when peeling a tag, expected commit, have %v", obj.Type())
+	}
+
+	// TODO: Should test a tag that annotates a different object than a commit
+	// but it's impossible at the moment to tag such an object.
+}