[BUG] Restrict when to make link absolute in markdown

- In markdown, links are proccessed to be made absolute against the
relevant base in that context. Such that `./src` will be transformed
into `http://example.com/owner/repo/src/branch/main/src`.
- Don't try to make the link absolute if the link has a schema that's
defined in `[markdown].CUSTOM_URL_SCHEMES`, because they can't be made
absolute and doing so could lead to problems (see test case, double
slash was transformed to single slash).
- Adds unit test.
- Resolves https://codeberg.org/Codeberg/Community/issues/1489

modules/markup/markdown/goldmark.go

@@ -7,6 +7,7 @@ import (
 	"bytes"
 	"fmt"
 	"regexp"
+	"slices"
 	"strings"
 
 	"code.gitea.io/gitea/modules/container"
@@ -129,11 +130,17 @@ func (g *ASTTransformer) Transform(node *ast.Document, reader text.Reader, pc pa
 		case *ast.Link:
 			// Links need their href to munged to be a real value
 			link := v.Destination
-			if len(link) > 0 && !markup.IsLink(link) &&
-				link[0] != '#' && !bytes.HasPrefix(link, byteMailto) {
-				// special case: this is not a link, a hash link or a mailto:, so it's a
-				// relative URL
 
+			// Do not process the link if it's not a link, starts with an hashtag
+			// (indicating it's an anchor link), starts with `mailto:` or any of the
+			// custom markdown URLs.
+			processLink := len(link) > 0 && !markup.IsLink(link) &&
+				link[0] != '#' && !bytes.HasPrefix(link, byteMailto) &&
+				!slices.ContainsFunc(setting.Markdown.CustomURLSchemes, func(s string) bool {
+					return bytes.HasPrefix(link, []byte(s+":"))
+				})
+
+			if processLink {
 				var base string
 				if ctx.IsWiki {
 					base = ctx.Links.WikiLink()