tpl/tplimpl: Fix template truth logic

Before this commit, due to a bug in Go's `text/template` package, this would print different output for typed nil interface values:

```
{{ if .AuthenticatedUser }}User is authenticated!{{ else }}{{ end }}
{{ if not .AuthenticatedUser }}{{ else }}}User is authenticated!{{ end }}
```

This commit works around this by wrapping every `if` and `with` with a custom `getif` template func with truth logic that matches `not`, `and` and `or`.

Those 3 template funcs from Go's stdlib are now pulled into Hugo's source tree and adjusted to support custom zero values, e.g. types that implement `IsZero`.

This means that you can now do:

```
{{ with .Date }}{{ . }}{{ end }}
```

And it would work as expected.

Fixes #5738

common/hreflect/helpers.go

@@ -0,0 +1,91 @@
+// Copyright 2019 The Hugo Authors. All rights reserved.
+// Some functions in this file (see comments) is based on the Go source code,
+// copyright The Go Authors and  governed by a BSD-style license.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package hreflect contains reflect helpers.
+package hreflect
+
+import (
+	"reflect"
+
+	"github.com/gohugoio/hugo/common/types"
+)
+
+// IsTruthful returns whether in represents a truthful value.
+// See IsTruthfulValue
+func IsTruthful(in interface{}) bool {
+	switch v := in.(type) {
+	case reflect.Value:
+		return IsTruthfulValue(v)
+	default:
+		return IsTruthfulValue(reflect.ValueOf(in))
+	}
+
+}
+
+var zeroType = reflect.TypeOf((*types.Zeroer)(nil)).Elem()
+
+// IsTruthfulValue returns whether the given value has a meaningful truth value.
+// This is based on template.IsTrue in Go's stdlib, but also considers
+// IsZero and any interface value will be unwrapped before it's considered
+// for truthfulness.
+//
+// Based on:
+// https://github.com/golang/go/blob/178a2c42254166cffed1b25fb1d3c7a5727cada6/src/text/template/exec.go#L306
+func IsTruthfulValue(val reflect.Value) (truth bool) {
+	val = indirectInterface(val)
+
+	if !val.IsValid() {
+		// Something like var x interface{}, never set. It's a form of nil.
+		return
+	}
+
+	if val.Type().Implements(zeroType) {
+		return !val.Interface().(types.Zeroer).IsZero()
+	}
+
+	switch val.Kind() {
+	case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
+		truth = val.Len() > 0
+	case reflect.Bool:
+		truth = val.Bool()
+	case reflect.Complex64, reflect.Complex128:
+		truth = val.Complex() != 0
+	case reflect.Chan, reflect.Func, reflect.Ptr, reflect.Interface:
+		truth = !val.IsNil()
+	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
+		truth = val.Int() != 0
+	case reflect.Float32, reflect.Float64:
+		truth = val.Float() != 0
+	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
+		truth = val.Uint() != 0
+	case reflect.Struct:
+		truth = true // Struct values are always true.
+	default:
+		return
+	}
+
+	return
+}
+
+// Based on: https://github.com/golang/go/blob/178a2c42254166cffed1b25fb1d3c7a5727cada6/src/text/template/exec.go#L931
+func indirectInterface(v reflect.Value) reflect.Value {
+	if v.Kind() != reflect.Interface {
+		return v
+	}
+	if v.IsNil() {
+		return reflect.Value{}
+	}
+	return v.Elem()
+}