Add basic fragment parsing function

This should work, but does no validation on the fragment after parsing.

gitdiff/gitdiff.go

@@ -35,8 +35,31 @@ type Fragment struct {
 
 	NewPosition int64
 	NewLines    int64
+
+	LinesAdded   int64
+	LinesDeleted int64
+
+	Lines []FragmentLine
 }
 
+// FragmentLine is a line in a fragment.
+type FragmentLine struct {
+	Op   LineOp
+	Line string
+}
+
+// LineOp describes the type of a fragment line: context, added, or removed.
+type LineOp int
+
+const (
+	// OpContext indicates a context line
+	OpContext LineOp = iota
+	// OpDelete indicates a deleted line
+	OpDelete
+	// OpAdd indicates an added line
+	OpAdd
+)
+
 // Header returns the cannonical header of this fragment.
 func (f *Fragment) Header() string {
 	return fmt.Sprintf("@@ -%d,%d +%d,%d @@ %s", f.OldPosition, f.OldLines, f.NewPosition, f.NewLines, f.Comment)