Fix copy/paste of empty lines (#19798)

* Fix copy/paste of empty newlines again

Fixes: https://github.com/go-gitea/gitea/issues/19331
Regressed by: https://github.com/go-gitea/gitea/pull/18270

Needed to do another newline addition to the Chroma output HTML to get
copy/paste work again. The previous replacement conditions are probably
obsolete, but as I'm not 100% sure, I opted to keep them.

Specifically, the Chroma HTML change mentioned in
https://github.com/go-gitea/gitea/pull/18270#issuecomment-1013350246
broke our previous newline replacement for such empty lines.

Also included are a few changes to make the test more pleasant to work
with.

* run go mod tidy

* add util.Dedent

* copy in the code

Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Lauris BH <lauris@nix.lv>

modules/util/util.go

@@ -9,6 +9,7 @@ import (
 	"crypto/rand"
 	"errors"
 	"math/big"
+	"regexp"
 	"strconv"
 	"strings"
 
@@ -191,3 +192,35 @@ var titleCaser = cases.Title(language.English)
 func ToTitleCase(s string) string {
 	return titleCaser.String(s)
 }
+
+var (
+	whitespaceOnly    = regexp.MustCompile("(?m)^[ \t]+$")
+	leadingWhitespace = regexp.MustCompile("(?m)(^[ \t]*)(?:[^ \t\n])")
+)
+
+// Dedent removes common indentation of a multi-line string along with whitespace around it
+// Based on https://github.com/lithammer/dedent
+func Dedent(s string) string {
+	var margin string
+
+	s = whitespaceOnly.ReplaceAllString(s, "")
+	indents := leadingWhitespace.FindAllStringSubmatch(s, -1)
+
+	for i, indent := range indents {
+		if i == 0 {
+			margin = indent[1]
+		} else if strings.HasPrefix(indent[1], margin) {
+			continue
+		} else if strings.HasPrefix(margin, indent[1]) {
+			margin = indent[1]
+		} else {
+			margin = ""
+			break
+		}
+	}
+
+	if margin != "" {
+		s = regexp.MustCompile("(?m)^"+margin).ReplaceAllString(s, "")
+	}
+	return strings.TrimSpace(s)
+}