Log Message:
mutt: Update to version 1.13.1

Changes since version 1.13.0:
! Bug fix release.
+ $sidebar_relative_shortpath_indent, default unset, enables the
  indentation and shortpath behavior introduced in 1.13.0.
+ $sidebar_use_mailbox_shortcuts, default unset, displays standard
  mailbox shortcuts, '~' and '=' in the sidebar.  When unset, the
  sidebar will remove a $folder prefix but won't display mailbox
  shortcuts.