Use git attributes to determine generated and vendored status for language stats and diffs (#16773)

Replaces #16262
Replaces #16250
Replaces #14833

This PR first implements a `git check-attr` pipe reader - using `git check-attr --stdin -z --cached` - taking account of the change in the output format in git 1.8.5 and creates a helper function to read a tree into a temporary index file for that pipe reader.

It then wires this in to the language stats helper and into the git diff generation.

Files which are marked generated will be folded by default.

Fixes #14786
Fixes #12653

modules/analyze/generated.go

@@ -0,0 +1,28 @@
+// Copyright 2021 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package analyze
+
+import (
+	"path/filepath"
+	"strings"
+
+	"github.com/go-enry/go-enry/v2/data"
+)
+
+// IsGenerated returns whether or not path is a generated path.
+func IsGenerated(path string) bool {
+	ext := strings.ToLower(filepath.Ext(path))
+	if _, ok := data.GeneratedCodeExtensions[ext]; ok {
+		return true
+	}
+
+	for _, m := range data.GeneratedCodeNameMatchers {
+		if m(path) {
+			return true
+		}
+	}
+
+	return false
+}