Add Tabular Diff for CSV files (#14661)

Implements request #14320 The rendering of CSV files does match the diff style.

* Moved CSV logic into base package.

* Added method to create a tabular diff.

* Added CSV compare context.

* Added CSV diff template.

* Use new table style in CSV markup.

* Added file size limit for CSV rendering.

* Display CSV parser errors in diff.

* Lazy read single file.

* Lazy read rows for full diff.

* Added unit tests for various CSV changes.

modules/csv/csv_test.go

@@ -0,0 +1,40 @@
+// 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 csv
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCreateReader(t *testing.T) {
+	rd := CreateReader([]byte{}, ',')
+	assert.Equal(t, ',', rd.Comma)
+}
+
+func TestCreateReaderAndGuessDelimiter(t *testing.T) {
+	input := "a;b;c\n1;2;3\n4;5;6"
+
+	rd := CreateReaderAndGuessDelimiter([]byte(input))
+	assert.Equal(t, ';', rd.Comma)
+}
+
+func TestGuessDelimiter(t *testing.T) {
+	var kases = map[string]rune{
+		"a":                         ',',
+		"1,2":                       ',',
+		"1;2":                       ';',
+		"1\t2":                      '\t',
+		"1|2":                       '|',
+		"1,2,3;4,5,6;7,8,9\na;b;c":  ';',
+		"\"1,2,3,4\";\"a\nb\"\nc;d": ';',
+		"<br/>":                     ',',
+	}
+
+	for k, v := range kases {
+		assert.EqualValues(t, guessDelimiter([]byte(k)), v)
+	}
+}