Fix hardlinks with unmodified files

Previously hardlinking to an unmodified file or linking to a file
which was touched by not detected as modified caused a new file
to be created on unpack. This new file and the original source file
were not linked since no link record was created in the tar. This
change addresses this by adding links for all hardlinks to a file
when it is detected as changed. These links will be written after
the source file is written and may occur out of order in regard to
file name.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

fs/diff.go

@@ -16,9 +16,13 @@ import (
 type ChangeKind int
 
 const (
+	// ChangeKindUnmodified represents an unmodified
+	// file
+	ChangeKindUnmodified = iota
+
 	// ChangeKindAdd represents an addition of
 	// a file
-	ChangeKindAdd = iota
+	ChangeKindAdd
 
 	// ChangeKindModify represents a change to
 	// an existing file
@@ -31,6 +35,8 @@ const (
 
 func (k ChangeKind) String() string {
 	switch k {
+	case ChangeKindUnmodified:
+		return "unmodified"
 	case ChangeKindAdd:
 		return "add"
 	case ChangeKindModify:
@@ -287,7 +293,10 @@ func doubleWalkDiff(ctx context.Context, changeFn ChangeFunc, a, b string) (err
 				f1 = nil
 				f2 = nil
 				if same {
-					continue
+					if !isLinked(f) {
+						continue
+					}
+					k = ChangeKindUnmodified
 				}
 			}
 			if err := changeFn(k, p, f, nil); err != nil {