Fix(helm): Fix the bug of dependency update deleting subcharts

In Helm 2.6.0, the new delete feature that deletes old versions of
charts is deleting subcharts that aren't in requirements.yaml.
In this patch, I judge the dependency whether it is included in the
requirements.yaml before deleting it.

Closes #2830

pkg/downloader/manager.go

@@ -217,9 +217,16 @@ func (m *Manager) downloadAll(deps []*chartutil.Dependency) error {
 	}
 
 	fmt.Fprintln(m.Out, "Deleting outdated charts")
+	c, _ := m.loadChartDir()
+	req, _ := chartutil.LoadRequirements(c)
 	for _, dep := range deps {
-		if err := m.safeDeleteDep(dep.Name, destPath); err != nil {
-			return err
+		for _, r := range req.Dependencies {
+			if dep.Name == r.Name {
+				if err := m.safeDeleteDep(dep.Name, destPath); err != nil {
+					return err
+				}
+				break
+			}
 		}
 	}