proper support for "isabelle update -D DIR": avoid accidental exclusion of select_dirs (amending e5dafe9e120f);
# Demo: Isabelle system component and Isabelle/Scala tool
This directory constitutes an Isabelle system component. It defines an
Isabelle/Scala tool that is available via the command-line as "isabelle demo".
See also "isabelle doc system" on "isabelle components -u" to register system
components in user-space.
NOTE: User-defined components should chose a name prefix that is unlikely to
clash with existing tools (or other user tools).