Add HasHead function to dataset (#3262)

go/datas/dataset.go

@@ -64,6 +64,11 @@ func (ds Dataset) MaybeHeadRef() (types.Ref, bool) {
 	return ds.headRef, ds.headRef != types.Ref{}
 }
 
+// HasHead() returns 'true' if this dataset has a Head Commit, false otherwise.
+func (ds Dataset) HasHead() bool {
+	return ds.headRef != types.Ref{}
+}
+
 // HeadRef returns the Ref of the current head Commit, which contains the
 // current root of the Dataset's value tree.
 func (ds Dataset) HeadRef() types.Ref {

go/datas/dataset_test.go

@@ -147,11 +147,13 @@ func TestHeadValueFunctions(t *testing.T) {
 	defer store.Close()
 
 	ds1 := store.GetDataset(id1)
+	assert.False(ds1.HasHead())
 
 	// ds1: |a|
 	a := types.String("a")
 	ds1, err := store.CommitValue(ds1, a)
 	assert.NoError(err)
+	assert.True(ds1.HasHead())
 
 	hv := ds1.Head().Get(ValueField)
 	assert.Equal(a, hv)