Set Setpgid on child git processes (#19865)

When Gitea is running as PID 1 git will occassionally orphan child processes leading
to (defunct) processes. This PR simply sets Setpgid to true on these child processes
meaning that these defunct processes will also be correctly reaped.

Fix #19077

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

modules/markup/external/external.go

@@ -124,6 +124,8 @@ func (p *Renderer) Render(ctx *markup.RenderContext, input io.Reader, output io.
 		cmd.Stdin = input
 	}
 	cmd.Stdout = output
+	process.SetSysProcAttribute(cmd)
+
 	if err := cmd.Run(); err != nil {
 		return fmt.Errorf("%s render run command %s %v failed: %v", p.Name(), commands[0], args, err)
 	}