Pass link prefixes to external markup parsers (#5201)

* Pass environment variables for URL prefixes to external markup parser

Signed-off-by: Nicolas Lenz <nicolas@eisfunke.com>

* Document external markup link prefix environment variables

Signed-off-by: Nicolas Lenz <nicolas@eisfunke.com>

* Run format on link prefix changes

Signed-off-by: Nicolas Lenz <nicolas@eisfunke.com>

modules/markup/external/external.go

@@ -76,6 +76,11 @@ func (p *Parser) Render(rawBytes []byte, urlPrefix string, metas map[string]stri
 	}
 
 	cmd := exec.Command(commands[0], args...)
+	cmd.Env = append(
+		os.Environ(),
+		"GITEA_PREFIX_SRC="+urlPrefix,
+		"GITEA_PREFIX_RAW="+strings.Replace(urlPrefix, "/src/", "/raw/", 1),
+	)
 	if !p.IsInputFile {
 		cmd.Stdin = rd
 	}