DirectoryHierarchy: UsedKeywords is a of the struct

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

check.go

@@ -8,7 +8,7 @@ package mtree
 // keywords) and then doing a Compare(dh, newDh, keywords).
 func Check(root string, dh *DirectoryHierarchy, keywords []string) ([]InodeDelta, error) {
 	if keywords == nil {
-		keywords = CollectUsedKeywords(dh)
+		keywords = dh.UsedKeywords()
 	}
 
 	newDh, err := Walk(root, nil, keywords)
@@ -25,7 +25,7 @@ func Check(root string, dh *DirectoryHierarchy, keywords []string) ([]InodeDelta
 // equivalent to Compare(dh, tarDH, keywords).
 func TarCheck(tarDH, dh *DirectoryHierarchy, keywords []string) ([]InodeDelta, error) {
 	if keywords == nil {
-		keywords = CollectUsedKeywords(dh)
+		keywords = dh.UsedKeywords()
 	}
 	return Compare(dh, tarDH, keywords)
 }

check_test.go

@@ -200,7 +200,7 @@ func TestTarTime(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	keywords := CollectUsedKeywords(dh)
+	keywords := dh.UsedKeywords()
 
 	// make sure "time" keyword works
 	_, err = Check(dir, dh, keywords)

cmd/gomtree/main.go

@@ -142,7 +142,7 @@ func app() error {
 
 		// We can't check against more fields than in the specKeywords list, so
 		// currentKeywords can only have a subset of specKeywords.
-		specKeywords = mtree.CollectUsedKeywords(specDh)
+		specKeywords = specDh.UsedKeywords()
 	}
 
 	// -list-used

hierarchy.go

@@ -26,13 +26,9 @@ func (dh DirectoryHierarchy) WriteTo(w io.Writer) (n int64, err error) {
 	return sum, nil
 }
 
-// CollectUsedKeywords collects and returns all the keywords used in a
+// UsedKeywords collects and returns all the keywords used in a
 // a DirectoryHierarchy
-func CollectUsedKeywords(dh *DirectoryHierarchy) []string {
-	if dh == nil {
-		return nil
-	}
-
+func (dh DirectoryHierarchy) UsedKeywords() []string {
 	usedkeywords := []string{}
 	for _, e := range dh.Entries {
 		switch e.Type {