format/diff: unified diff encoder and public API

COMPATIBILITY.md

@@ -39,7 +39,7 @@ is supported by go-git.
 | **patching** |
 | apply                                 | ✖ |
 | cherry-pick                           | ✖ |
-| diff                                  | ✖ |
+| diff                                  | ✔ | Patch object with UnifiedDiff output representation |
 | rebase                                | ✖ |
 | revert                                | ✖ |
 | **debugging** |

plumbing/format/diff/patch.go

@@ -0,0 +1,58 @@
+package diff
+
+import (
+	"gopkg.in/src-d/go-git.v4/plumbing"
+	"gopkg.in/src-d/go-git.v4/plumbing/filemode"
+)
+
+// Operation defines the operation of a diff item.
+type Operation int
+
+const (
+	// Equal item represents a equals diff.
+	Equal Operation = iota
+	// Add item represents an insert diff.
+	Add
+	// Delete item represents a delete diff.
+	Delete
+)
+
+// Patch represents a collection of steps to transform several files.
+type Patch interface {
+	// FilePatches returns a slice of patches per file.
+	FilePatches() []FilePatch
+	// Message returns an optional message that can be at the top of the
+	// Patch representation.
+	Message() string
+}
+
+// FilePatch represents the necessary steps to transform one file to another.
+type FilePatch interface {
+	// IsBinary returns true if this patch is representing a binary file.
+	IsBinary() bool
+	// Files returns the from and to Files, with all the necessary metadata to
+	// about them. If the patch creates a new file, "from" will be nil.
+	// If the patch deletes a file, "to" will be nil.
+	Files() (from, to File)
+	// Chunks returns a slice of ordered changes to transform "from" File to
+	// "to" File. If the file is a binary one, Chunks will be empty.
+	Chunks() []Chunk
+}
+
+// File contains all the file metadata necessary to print some patch formats.
+type File interface {
+	// Hash returns the File Hash.
+	Hash() plumbing.Hash
+	// Mode returns the FileMode.
+	Mode() filemode.FileMode
+	// Path returns the complete Path to the file, including the filename.
+	Path() string
+}
+
+// Chunk represents a portion of a file transformation to another.
+type Chunk interface {
+	// Content contains the portion of the file.
+	Content() string
+	// Type contains the Operation to do with this Chunk.
+	Type() Operation
+}