format/packfile: implement delta encoding (#172)

* format/packfile: implement delta encoding

- Added all the logic to the encoder to be able to encode ref-delta and offset-delta objects
- Created plumbing.ObjectToPack to handle deltas and standard objects when we are writting them into a packfile
- Added specific encoder delta tests, one standard object and one delta, and one standard object and two deltas

* Requested changes.

* Requested changes

1 files changed, 40 insertions, 0 deletions

diff --git a/plumbing/format/packfile/object_pack_test.go b/plumbing/format/packfile/object_pack_test.go
new file mode 100644
index 0000000..ddc7ab5
--- /dev/null
+++ b/plumbing/format/packfile/object_pack_test.go
@@ -0,0 +1,40 @@
+package packfile
+
+import (
+	"io"
+
+	"gopkg.in/src-d/go-git.v4/plumbing"
+
+	. "gopkg.in/check.v1"
+)
+
+type ObjectToPackSuite struct{}
+
+var _ = Suite(&ObjectToPackSuite{})
+
+func (s *ObjectToPackSuite) TestObjectToPack(c *C) {
+	obj := &dummyObject{}
+	otp := newObjectToPack(obj)
+	c.Assert(obj, Equals, otp.Object)
+	c.Assert(obj, Equals, otp.Original)
+	c.Assert(otp.Base, IsNil)
+	c.Assert(otp.IsDelta(), Equals, false)
+
+	original := &dummyObject{}
+	delta := &dummyObject{}
+	deltaToPack := newDeltaObjectToPack(otp, original, delta)
+	c.Assert(obj, Equals, deltaToPack.Object)
+	c.Assert(original, Equals, deltaToPack.Original)
+	c.Assert(otp, Equals, deltaToPack.Base)
+	c.Assert(deltaToPack.IsDelta(), Equals, true)
+}
+
+type dummyObject struct{}
+
+func (*dummyObject) Hash() plumbing.Hash             { return plumbing.ZeroHash }
+func (*dummyObject) Type() plumbing.ObjectType       { return plumbing.InvalidObject }
+func (*dummyObject) SetType(plumbing.ObjectType)     {}
+func (*dummyObject) Size() int64                     { return 0 }
+func (*dummyObject) SetSize(s int64)                 {}
+func (*dummyObject) Reader() (io.ReadCloser, error)  { return nil, nil }
+func (*dummyObject) Writer() (io.WriteCloser, error) { return nil, nil }