Refactor changes and test functions

Remove change type in favor of explicit change function.
Using change function makes it more difficult to unnecessarily
add to the change interface.

Update test apply functions to use an interface rather
than a function type.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)

fs/path.go

@@ -15,42 +15,27 @@ type currentPath struct {
 	fullPath string
 }
 
-func pathChange(lower, upper *currentPath) Change {
+func pathChange(lower, upper *currentPath) (ChangeKind, string) {
 	if lower == nil {
 		if upper == nil {
 			panic("cannot compare nil paths")
 		}
-		return Change{
-			Kind: ChangeKindAdd,
-			Path: upper.path,
-		}
+		return ChangeKindAdd, upper.path
 	}
 	if upper == nil {
-		return Change{
-			Kind: ChangeKindDelete,
-			Path: lower.path,
-		}
+		return ChangeKindDelete, lower.path
 	}
 	// TODO: compare by directory
 
 	switch i := strings.Compare(lower.path, upper.path); {
 	case i < 0:
 		// File in lower that is not in upper
-		return Change{
-			Kind: ChangeKindDelete,
-			Path: lower.path,
-		}
+		return ChangeKindDelete, lower.path
 	case i > 0:
 		// File in upper that is not in lower
-		return Change{
-			Kind: ChangeKindAdd,
-			Path: upper.path,
-		}
+		return ChangeKindAdd, upper.path
 	default:
-		return Change{
-			Kind: ChangeKindModify,
-			Path: upper.path,
-		}
+		return ChangeKindModify, upper.path
 	}
 }