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, 16 insertions, 0 deletions

diff --git a/utils/binary/writer_test.go b/utils/binary/writer_test.go
index 88140a1..1380280 100644
--- a/utils/binary/writer_test.go
+++ b/utils/binary/writer_test.go
@@ -41,3 +41,19 @@ func (s *BinarySuite) TestWriteUint16(c *C) {
 	c.Assert(err, IsNil)
 	c.Assert(buf, DeepEquals, expected)
 }
+
+func (s *BinarySuite) TestWriteVariableWidthInt(c *C) {
+	buf := bytes.NewBuffer(nil)
+
+	err := WriteVariableWidthInt(buf, 366)
+	c.Assert(err, IsNil)
+	c.Assert(buf.Bytes(), DeepEquals, []byte{129, 110})
+}
+
+func (s *BinarySuite) TestWriteVariableWidthIntShort(c *C) {
+	buf := bytes.NewBuffer(nil)
+
+	err := WriteVariableWidthInt(buf, 19)
+	c.Assert(err, IsNil)
+	c.Assert(buf.Bytes(), DeepEquals, []byte{19})
+}