Links in markdown should be absolute to the repository not the server (#15088)

* Links in markdown should be absolute to the repository not the server

Fix #15075

Signed-off-by: Andrew Thornton <art27@cantab.net>

* match github

Signed-off-by: Andrew Thornton <art27@cantab.net>

* add testcase

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-authored-by: 6543 <6543@obermui.de>

modules/markup/markdown/goldmark.go

@@ -98,7 +98,8 @@ func (g *ASTTransformer) Transform(node *ast.Document, reader text.Reader, pc pa
 				}
 				prefix = strings.Replace(prefix, "/src/", "/media/", 1)
 
-				lnk := string(link)
+				lnk := strings.TrimLeft(string(link), "/")
+
 				lnk = giteautil.URLJoin(prefix, lnk)
 				link = []byte(lnk)
 			}

modules/markup/markdown/markdown_test.go

@@ -93,6 +93,19 @@ func TestRender_Images(t *testing.T) {
 	test(
 		"[!["+title+"]("+url+")]("+href+")",
 		`<p><a href="`+href+`" rel="nofollow"><img src="`+result+`" alt="`+title+`"/></a></p>`)
+
+	url = "/../../.images/src/02/train.jpg"
+	test(
+		"!["+title+"]("+url+")",
+		`<p><a href="`+result+`" rel="nofollow"><img src="`+result+`" alt="`+title+`"/></a></p>`)
+
+	test(
+		"[["+title+"|"+url+"]]",
+		`<p><a href="`+result+`" rel="nofollow"><img src="`+result+`" title="`+title+`" alt="`+title+`"/></a></p>`)
+	test(
+		"[!["+title+"]("+url+")]("+href+")",
+		`<p><a href="`+href+`" rel="nofollow"><img src="`+result+`" alt="`+title+`"/></a></p>`)
+
 }
 
 func testAnswers(baseURLContent, baseURLImages string) []string {