Valid email address should only start with alphanumeric (#28174)

This fixes issue #27847 where regular expression allowed email address
to start with special symbols. Valid email addresses should start with
alphanumeric character, and as such will be rendered as email.

Added test cases from the bug report to validate, such input will not be
rendered anymore as email address.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/markup/html_email.go

@@ -3,7 +3,11 @@
 
 package markup
 
-import "golang.org/x/net/html"
+import (
+	"strings"
+
+	"golang.org/x/net/html"
+)
 
 // emailAddressProcessor replaces raw email addresses with a mailto: link.
 func emailAddressProcessor(ctx *RenderContext, node *html.Node) {
@@ -14,6 +18,14 @@ func emailAddressProcessor(ctx *RenderContext, node *html.Node) {
 			return
 		}
 
+		var nextByte byte
+		if len(node.Data) > m[3] {
+			nextByte = node.Data[m[3]]
+		}
+		if strings.IndexByte(":/", nextByte) != -1 {
+			// for cases: "git@gitea.com:owner/repo.git", "https://git@gitea.com/owner/repo.git"
+			return
+		}
 		mail := node.Data[m[2]:m[3]]
 		replaceContent(node, m[2], m[3], createLink(ctx, "mailto:"+mail, mail, "" /*mailto*/))
 		node = node.NextSibling.NextSibling