format: diff, remove error when two files are empty, submodules are simply ignored

Signed-off-by: Máximo Cuadros <mcuadros@gmail.com>

Author: mcuadros

plumbing/format/diff/unified_encoder.go

@@ -2,7 +2,6 @@ package diff
 
 import (
 	"bytes"
-	"errors"
 	"fmt"
 	"io"
 	"strings"
@@ -45,8 +44,6 @@ const (
 	DefaultContextLines = 3
 )
 
-var ErrBothFilesEmpty = errors.New("both files are empty")
-
 // UnifiedEncoder encodes an unified diff into the provided Writer.
 // There are some unsupported features:
 //     - Similarity index for renames
@@ -106,7 +103,7 @@ func (e *UnifiedEncoder) printMessage(message string) {
 func (e *UnifiedEncoder) header(from, to File, isBinary bool) error {
 	switch {
 	case from == nil && to == nil:
-		return ErrBothFilesEmpty
+		return nil
 	case from != nil && to != nil:
 		hashEquals := from.Hash() == to.Hash()
 

plumbing/format/diff/unified_encoder_test.go

@@ -20,7 +20,7 @@ func (s *UnifiedEncoderTestSuite) TestBothFilesEmpty(c *C) {
 	buffer := bytes.NewBuffer(nil)
 	e := NewUnifiedEncoder(buffer, 1)
 	err := e.Encode(testPatch{filePatches: []testFilePatch{{}}})
-	c.Assert(err, Equals, ErrBothFilesEmpty)
+	c.Assert(err, IsNil)
 }
 
 func (s *UnifiedEncoderTestSuite) TestBinaryFile(c *C) {