compare: allow nil newDh and oldDh

This allows people to create synthetic InodeDeltas, which is something
that umoci would like to be able to do in order to nicely create 'umoci
insert' layers.

Signed-off-by: Aleksa Sarai <asarai@suse.de>

compare.go

@@ -325,6 +325,10 @@ func compareEntry(oldEntry, newEntry Entry) ([]KeyDelta, error) {
 // missing entries and the like). A missing or extra key is treated as a
 // Modified type.
 //
+// If oldDh or newDh are empty, we assume they are a hierarchy that is
+// completely empty. This is purely for helping callers create synthetic
+// InodeDeltas.
+//
 // NB: The order of the parameters matters (old, new) because Extra and
 //     Missing are considered as different discrepancy types.
 func Compare(oldDh, newDh *DirectoryHierarchy, keys []Keyword) ([]InodeDelta, error) {
@@ -338,7 +342,8 @@ func Compare(oldDh, newDh *DirectoryHierarchy, keys []Keyword) ([]InodeDelta, er
 	// map to make sure we don't start comparing unrelated entries.
 	diffs := map[string]*stateT{}
 
-	// First, iterate over the old hierarchy.
+	// First, iterate over the old hierarchy. If nil, pretend it's empty.
+	if oldDh != nil {
 		for _, e := range oldDh.Entries {
 			if e.Type == RelativeType || e.Type == FullType {
 				path, err := e.Path()
@@ -357,8 +362,10 @@ func Compare(oldDh, newDh *DirectoryHierarchy, keys []Keyword) ([]InodeDelta, er
 				diffs[path].Old = copy
 			}
 		}
+	}
 
-	// Then, iterate over the new hierarchy.
+	// Then, iterate over the new hierarchy. If nil, pretend it's empty.
+	if newDh != nil {
 		for _, e := range newDh.Entries {
 			if e.Type == RelativeType || e.Type == FullType {
 				path, err := e.Path()
@@ -377,6 +384,7 @@ func Compare(oldDh, newDh *DirectoryHierarchy, keys []Keyword) ([]InodeDelta, er
 				diffs[path].New = copy
 			}
 		}
+	}
 
 	// Now we compute the diff.
 	var results []InodeDelta