detect when backend.Hash needs update

It's possible to not change the backend config, but require updating the
stored backend state by moving init options from the config file to the
`-backend-config` flag. If the config is the same, but the hash doesn't
match, update the stored state.

command/init_test.go

@@ -464,6 +464,50 @@ func TestInit_backendReinitWithExtra(t *testing.T) {
 	}
 }
 
+// move option from config to -backend-config args
+func TestInit_backendReinitConfigToExtra(t *testing.T) {
+	td := tempDir(t)
+	copy.CopyDir(testFixturePath("init-backend"), td)
+	defer os.RemoveAll(td)
+	defer testChdir(t, td)()
+
+	ui := new(cli.MockUi)
+	c := &InitCommand{
+		Meta: Meta{
+			ContextOpts: testCtxConfig(testProvider()),
+			Ui:          ui,
+		},
+	}
+
+	if code := c.Run([]string{"-input=false"}); code != 0 {
+		t.Fatalf("bad: \n%s", ui.ErrorWriter.String())
+	}
+
+	// Read our saved backend config and verify we have our settings
+	state := testStateRead(t, filepath.Join(DefaultDataDir, DefaultStateFilename))
+	if v := state.Backend.Config["path"]; v != "foo" {
+		t.Fatalf("bad: %#v", v)
+	}
+
+	backendHash := state.Backend.Hash
+
+	// init again but remove the path option from the config
+	cfg := "terraform {\n  backend \"local\" {}\n}\n"
+	if err := ioutil.WriteFile("main.tf", []byte(cfg), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	args := []string{"-input=false", "-backend-config=path=foo"}
+	if code := c.Run(args); code != 0 {
+		t.Fatalf("bad: \n%s", ui.ErrorWriter.String())
+	}
+	state = testStateRead(t, filepath.Join(DefaultDataDir, DefaultStateFilename))
+
+	if state.Backend.Hash == backendHash {
+		t.Fatal("state.Backend.Hash was not updated")
+	}
+}
+
 /*
 func TestInit_remoteState(t *testing.T) {
 	tmp, cwd := testCwd(t)

command/meta_backend.go

@@ -1158,6 +1158,16 @@ func (m *Meta) backend_C_r_S_unchanged(
 	c *config.Backend, sMgr state.State) (backend.Backend, error) {
 	s := sMgr.State()
 
+	// it's possible for a backend to be unchanged, and the config itself to
+	// have changed by moving a paramter from the config to `-backend-config`
+	// In this case we only need to update the Hash.
+	if c != nil && s.Backend.Hash != c.Hash {
+		s.Backend.Hash = c.Hash
+		if err := sMgr.WriteState(s); err != nil {
+			return nil, fmt.Errorf(errBackendWriteSaved, err)
+		}
+	}
+
 	// Create the config. We do this from the backend state since this
 	// has the complete configuration data whereas the config itself
 	// may require input.